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DISEASES OF THE HORSE'S FOOT
By H. CAULTON REEKS
Fellow of the Royal College of Veterinary Surgeons Author of 'The Common Colics of the Horse'
1906
To J. MacQueen, F.R.C.V.S.,
Professor of Surgery at the Royal Veterinary College, London, as a slight acknowledgment of his ability as a teacher, and in return for many kindly services, this volume is gratefully inscribed by
THE AUTHOR.
PREFACE
Stimulated by the reception accorded my 'Common Colics of the Horse,' both in this country and in America, and assured by my publishers that a work on diseases of the foot was needed, I have been led to give to the veterinary profession the present volume.
While keeping the size of the book within reasonable limits, no effort has been spared to render it as complete as possible. This has only been achieved by adding to my own experience a great deal of the work of others. To mention individually those who have given me permission to use their writings would be too long a matter here. In every case, however, where the quotation is of any length, the source of my information is given, either in the text or in an accompanying footnote. A few there are who will, perhaps, find themselves quoted without my having first obtained their permission to do so. They, with the others, will, I am sure, accept my hearty thanks.
The publishers have been generous in the matter of illustrations and diagrams, and although to the older practitioner some of these may appear superfluous, it is hoped they will serve to render the work an acceptable textbook for the student.
H. CAULTON REEKS.
SPALDING, January, 1906.
CONTENTS
CHAPTER I
INTRODUCTION
CHAPTER II
REGIONAL ANATOMY
A. The Bones B. The Ligaments C. The Tendons D. The Arteries E. The Veins F. The Nerves G. The Complementary Apparatus of the Os Pedis H. The Keratogenous Membrane I. The Hoof
CHAPTER III
GENERAL PHYSIOLOGICAL AND ANATOMICAL OBSERVATIONS
A. Development of the Hoof B. Chemical Properties and Histology of Horn C. Expansion and Contraction of the Hoof D. The Functions of the Lateral Cartilages E. Growth of the Hoof
CHAPTER IV
METHOD OF EXAMINING THE FOOT
CHAPTER V
GENERAL REMARKS ON OPERATIONS ON THE FOOT
A. Methods of Restraint B. Instruments required C. The Application of Dressings D. Plantar Neurectomy History of the Operation Preparation of the Subject The Operation After-treatment E. Median Neurectomy F. Length of Rest after Neurectomy G. Sequelae of Neurectomy Liability of Pricked Foot going undetected Loss of Tone in the Non-sensitive Area Gelatinous Degeneration Chronic Oedema of the Leg Persistent Pruritus Fracture of the Bones Neuroma Reunion of the Divided Nerve The Existence of an Adventitious Nerve-supply Stumbling H. Advantages of the Operation I. The Use of the Horse that has undergone Neurectomy
CHAPTER VI
FAULTY CONFORMATION
A. Weak Heels B. Contracted Foot (a) Contracted Heels (b) Local or Coronary Contraction C. Flat-foot D. Pumiced-foot, Dropped Sole, or Convex Sole E. 'Ringed' or 'Ribbed' Hoof F. The Hoof with Bad Horn (a) The Brittle Hoof (b) The Spongy Hoof G. Club-Foot H. The Crooked Foot (a) The Foot with Unequal Sides (b) The Curved Hoof
CHAPTER VII
DISEASES ARISING FROM FAULTY CONFORMATION
A. Sand-crack Definition Classification Causes Complications Treatment Surgical Shoeing for Sand-crack
B. Corns Definition Classification Causes Pathological Anatomy and Histology Treatment Surgical Shoeing for Corn
C. Chronic Bruised Sole
CHAPTER VIII
WOUNDS OF THE KERATOGENOUS MEMBRANE
A. Nail-bound Definition Causes Symptoms Treatment
B. Punctured Foot Definition Causes Common Situations of the Wound Classification Symptoms and Diagnosis Complications Prognosis Treatment
C. Coronitis (Simple)
1. Acute Definition Causes Symptoms Complications Prognosis Treatment
2. Chronic Definition Causes Symptoms Treatment
D. False Quarter Definition Causes Treatment
E. Accidental Tearing off of the Entire Hoof
CHAPTER IX
INFLAMMATORY AFFECTIONS OF THE KERATOGENOUS APPARATUS
A. ACUTE Acute Laminitis Definition Causes Symptoms Pathological Anatomy Complications Diagnosis and Prognosis Treatment Broad's Treatment for Laminitis Smith's Operation for Laminitis
B. CHRONIC
1. Chronic Laminitis Definition Causes Symptoms Pathological Anatomy Treatment
2. Seedy-Toe Definition Causes Symptoms Treatment
3. Keraphyllocele Definition Causes Symptoms Treatment
4. Keratoma
5. Thrush Definition Causes Symptoms Treatment
6. Canker Definition Causes, Predisposing and Exciting Symptoms and Pathological Anatomy Differential Diagnosis and Prognosis Treatment Malcolm's, Lieutenant Rose's, Bermbach's, Hoffmann's and Imminger's Treatment for Canker
7. Specific Coronitis Definition Causes Symptoms Treatment
CHAPTER X
DISEASES OF THE LATERAL CARTILAGES
A. Wounds of the Cartilages
B. Quittor Definition Classification
1. Simple or Cutaneous Quittor Definition Causes Symptoms Pathological Anatomy Prognosis Complications Treatment, Preventive and Curative
2. Sub-horny Quittor Definition Causes Symptoms and Diagnosis Complications Necrosis of the Lateral Cartilage Pathological Anatomy of the Diseased Cartilage Necrosis of Tendon and of Ligament Ossification of the Cartilage Treatment Operations for Extirpation of the Cartilage
C. Ossification of the Lateral Cartilages (Side-bones) Definition Symptoms and Diagnosis Causes Treatment Smith's Operation for Ossification of the Lateral Cartilages
CHAPTER XI
DISEASES OF THE BONES
A. Periostitis and Ostitis 1. Periostitis (a) Simple Acute Periostitis (b) Suppurative Periostitis (c) Osteoplastic Periostitis
2. Ostitis (a) Rarefying Ostitis (b) Osteoplastic Ostitis (c) Caries and Necrosis
Treatment of Periostitis
Recorded Cases of Periostitis
B. Pyramidal Disease, Buttress Foot, or Low Ringbone Definition Symptoms and Diagnosis Pathological Anatomy Treatment Recorded Cases of Buttress Foot
C. Fractures of the Bones 1. Fractures of the Os Coronae Recorded Cases of Fractures of the Os Coronae
2. Fractures of the Os Pedis Recorded Cases of Fractures of the Os Pedis
3. Fractures of the Navicular Bone Recorded Case of Fracture of the Navicular Bone
Treatment of Fractures of the Bones of the Foot
CHAPTER XII
DISEASES OF THE JOINTS
A. Synovitis (a) Simple (1) Acute (2) Chronic (b) Purulent or Suppurative B. Arthritis (a) Simple or Serous (b) Acute (c) Purulent or Suppurative (d) Anchylosis
C. Navicular Disease Definition History Pathology Changes in the Bursa Changes in the Cartilage Changes in the Tendon Changes in the Bone Causes Heredity Compression Concussion A Weak Navicular Bone An Irregular Blood-supply to the Bone Senile Decay Symptoms and Diagnosis Differential Diagnosis Prognosis Treatment
D. Dislocations
LIST OF ILLUSTRATIONS
1. The Bones of the Phalanx 2. The Os Coronae (Anterior View) 3. The Os Coronae (Posterior View) 4. The Os Pedis (Postero-lateral View) 5. The Os Pedis (viewed from Below) 6. The Navicular Bone (viewed from Below) 7. The Navicular Bone (viewed from Above) 8. Ligaments of the First and Second Interphalangeal Articulations (Lateral View). (After Dollar and Wheatley) 9. Ligaments of the First and Second Interphalangeal Articulations (viewed from Behind). (After Dollar and Wheatley) 10. The Flexor Tendons and the Extensor Pedis. (After Hauebner) 11. The Flexor Perforans and Perforatus 12. The Flexor Perforans and Perforatus (the Perforans cut through and deflected) 13. Median Section of Normal Foot 14. The Arteries of the Foot 15. The Veins and Nerves of the Foot 16. The Lateral Cartilage 17. The Keratogenous Membrane (viewed from the Side) 18. The Keratogenous Membrane (viewed from Below) 19. The Wall of the Hoof 20. Internal Features of the Hoof 21. Inferior Aspect of the Hoof 22. Hoof with the Sensitive Structures removed 23. Section of Epidermis 24. Section of Skin with Hair Follicle and Hair 25. Section of Human Nail and Nail-bed 26. Section of Foot of Equine Foetus. (Mettam) 27. Section from Foot of Sheep Embryo. (Mettam) 28. Section from Foot of Calf Embryo. (Mettam) 29. Section from Foot of Equine Foetus. (Mettam) 30. Section through Hoof and Soft Tissues of a Foal at Term. (Mettam) 31. Perpendicular Section of Horn of Wall 32. Horizontal Section of Horn of Wall 33. Horizontal Section through the Junction of the Wall with the Sole 34. Section of Frog. (Mettam) 35. Professor Lungwitz's Apparatus for Examining the Foot Movements 36. Professor Lungwitz's Apparatus for Examining the Foot Movements 37. The Movements of the Solar and Coronary Edges of the Hoof illustrated. (Lungwitz) 38. The Blind 39. The Side-line 40. Method of securing the Hind-foot with the Side-line 41. The Hind-foot secured with the Side-line 42. The Casting Hobbles 43. Method of securing the Hind-leg upon the Fore 44. The Hind-leg secured upon the Fore 45. The Drawing-knife (Ordinary Pattern) 46. Modern Forms of Drawing-knives 47. Symes's Knife 48-51. Illustrating Colonel Nunn's Method of applying a Poultice to the Foot 52. Poultice-boot of Canvas and Steel 53. Poultice-boot of Cocoa-fibre 54. Foot-swab 55. The Shoe with Plates 56. Quittor Syringe 57. The Esmarch Bandage and Tourniquet 58. Tourniquet with Wooden Block 59. Neurectomy Bistoury 60. Neurectomy Needle 61. Double Neurectomy Tenaculum 62. Adventitious Nerve-supply to Foot. (Sessions) 63. Tip Shoe 64. The Tip Shoe 'let in' to the Foot 65. The Thinned Tip 66. Drawing-knife for Charlier Shoeing 67. The Foot prepared for the Charlier Shoe 68. Bar Shoe 69. Rubber Bar Pad on Leather 70. The Bar Pad applied with a Half-shoe 71. Frog Pad 72. Frog Pad applied 73. Smith's Expansion Shoe for Contracted Feet 74. A Contracted Foot treated with Smith's Shoe 75. De Fay's Vice 76. Hartmann's Expanding Shoe 77. Broue's Slipper Shoe. (Gutenacker) 78. Einsiedel's Slipper and Bar-clip Shoe. (Gutenacker) 79. Hoof showing Coronary Contraction. (Gutenacker) 80. Flat-foot (Solar Surface). (Gutenacker) 81. Hoof showing Laminitis Rings on the Wall. (Gutenacker) 82. Hoof showing 'Grass' Rings on the Wall. (Gutenacker) 83. Club-foot. (Gutenacker) 84. Shoe with extended Toe-piece. (Gutenacker) 85. A Crooked Foot in Cross-section. (Gutenacker) 86. Sand-crack Firing-iron 87. Sand-crack Forceps and Clamp. (Vachette's) 88. McGill's Sand-crack Clamp 89. Koster's Sand-crack Clamp 90. Sand-crack Belt 91. Method of 'Easing' the Bearing of the Wall on the Shoe in the Treatment of Sand-crack 92. Method of 'Easing' the Bearing of the Wall on the Shoe in the Treatment of Sand-crack 93. Method of 'Easing' the Bearing of the Wall on the Shoe in the Treatment of Sand-crack 94 96. Grooving the Wall in the Treatment of Sand-crack 97. Removing the Wall in the Treatment of Sand-crack 98. Removing the Wall in the Treatment of Sand-crack 99. Horizontal Section of Corn. (Gutenacker) 100. Inner Surface of the Wall, showing Changes in Chronic Corn. (Gutenacker) 101. Perpendicular Section of the Wall in a Case of Chronic Corn. (Gutenacker) 102. Three-quarter Shoe 103. Three-quarter Bar Shoe 104. Shoe with a 'Dropped' Heel 105. Shoe with a 'Set' Heel 106. Curette, or Volkmann's Spoon 107. Resection of the Terminal Portion of the Perforans Tendon (Gutenacker) 108. Shoe with extended Toe-piece. (Colonel Nunn) 109. Mesian Section of Foot with Lesions following Coronitis. (Gutenacker) 110. Toe of Ordinary Hind-shoe 111. Toe of Hind-shoe Bevelled for the Prevention of Overreach 112. Hoof showing Lesion in the Wall following Coronitis. (Gutenacker) 113. Foot with Lesions of Chronic Coronitis. (Gutenacker) 114. Hoof Accidentally Tom from Foot. (Cartledge) 115. Hoof Accidentally Tom from Foot. (Rogerson) 116. Section of Foot with Laminitis of Eight Days' Duration. (Gutenacker) 117. Section of Foot with Laminitis of Fourteen Days' Duration. (Gutenacker) 118. Chronic Ostitis of the Os Pedis in Laminitis. 119. Broad's Rocker Bar Shoe for Laminitis. 120. The Foot showing Grooves made in the Wall for Treatment of Laminitis (Anterior Surface). 121. The Foot showing Grooves made for the Treatment of Laminitis (Solar Surface). 122. Foot with Chronic Laminitis. (Gutenacker) 123. Inferior Aspect of Foot with Chronic Laminitis. (Gutenacker) 124. Section of Foot with Laminitis of Three Weeks' Duration. (Gutenacker) 125. Section of Foot with Laminitis of Several Years' Duration. (Gutenacker) 126. Diagram showing Position of the Abnormal Growth of Horn in Chronic Laminitis. 127. Diagram showing the same Abnormal Growth of Horn Removed prior to Shoeing. 128. Shoe with Heel-clip. 129. Internal Seedy-Toe. 130. External Seedy-Toe. (Colonel Nunn) 131. External Seedy-Toe. (Colonel Nunn) 132. A Keraphyllocele on the Inner Surface of the Horn of the Wall at the Toe. (Gutenacker) 133. Os Pedis showing Absorption of Bone caused by the Pressure of a Keraphyllocele. (Gutenacker) 134. Foot with Canker of the Frog and Heels. (Gutenacker) 135. Foot with Canker extending to the Wall. (Malcolm) 136. Foot with Advanced Canker. (Gutenacker) 137. Feet affected with Specific Coronitis. (Taylor) 138. Fore-foot with Specific Coronitis. (Taylor) 139. Excision of the Lateral Cartilage (Old Method). (Gutenacker) 140. Excision of the Lateral Cartilage. (After Moller and Frick). (Gutenacker) 141. Excision of the Lateral Cartilage. (After Bayer.) (Gutenacker) 142. Partial Excision of the Lateral Cartilage 143. Ossified Lateral Cartilages, or Side-bones. 144. Smith's Side-bone Saw (Old Pattern). 145. Smith's Side-bone Saw (Improved Pattern). 146. Smith's Hoof Plane. 147. Hodder's Hoof Chisel. 148. Foot showing the Grooves made in Smith's Operation for Side-bones (viewed from the Side). 149. Foot showing the Grooves made in Smith's Operation for Side-bones (viewed from Below). 150. Periostitis involving the Pedal and Navicular Bones. (Litt) 151. Periostitis involving the Pedal and Navicular Bones. (Litt) 152. Effects of Periostitis on the Os Pedis. (Smith) 153. Effects of Periostitis on the Os Pedis. (Smith) 154. Effects of Periostitis on the Os Pedis. (Jones) 155. Effects of Periostitis on the Os Pedis. (Jones) 156. Case of Buttress Foot. (Routledge) 157. Foot showing Fracture of the Pyramidal Process in a Case of Buttress Foot. (Routledge) 158. Fracture of the Os Coronae. (Crawford) 159. Fracture of the Os Coronae. (Crawford) 160. Fractured Os Pedis. (Freeman) 161. Navicular Bone showing Lesions of Navicular Disease. (Gutenacker) 162. Foot with the Seat of Navicular Disease exposed (showing Lesions). (Gutenacker) 163. Navicular Bone showing Lesions of Navicular Disease (a Case of Long-standing). (Gutenacker) 164. Frog Seton Needle. 165. Diagram showing Course of the Needle in Setoning the Frog.
DISEASES OF THE HORSE'S FOOT
CHAPTER I
INTRODUCTION
The importance of that branch of veterinary surgery dealing with diseases of the horse's foot can hardly be overestimated. That the animal's usefulness is dependent upon his possession of four good feet is a fact that has long been recognised. Who, indeed, is there to be found entirely unacquainted with one or other of such well-known aphorisms as: 'Whoever hath charge of a horse's foot has the care of his whole body'; 'As well a horse with no head as a horse with no foot'; or the perhaps better known, and certainly more epigrammatic, 'No foot, no horse.'
Without taking these sayings literally, it will be admitted by almost everyone that they contain a vast amount of actual truth. This allowed, it at once becomes clear that a ready understanding of the diseases to which the foot is liable, the means of holding them in check, and the correct methods of treating them should figure largely in the knowledge at the command of the veterinary surgeon.
In the very great majority of instances the horse's ability to perform labour is the one thing that justifies his existence, and to that end the presence of four good, sound feet is an almost indispensable qualification. And yet how many circumstances do we see tending to militate against that one essential.
Even in colthood the foot, if neglected, may become a source of trouble. Unless periodically examined and properly trimmed, its shape is liable to serious alteration. From that in which it is best calculated to withstand the effects of the wear it will be called upon to endure in after life, it may become so changed for the worse as to seriously affect the animal's value.
In the matter of feeding, too, trouble is likely to ensue. Particularly is this the case where the colt shows points of exceptional merit. He is 'got up' for show, and the feet are likely to fall victims to the mismanagement that frequent exhibition so often carries with it. An extra allowance of peas, beans, wheat, or other equally injurious food is given. The result is a severe attack of laminitis, and an otherwise valuable and promising colt is permanently ruined.
Exposed as it is, too, to injury, the foot of a young horse, even at grass, is frequently the seat of injuries from picked up nails, stakes, or other agents which, unless detected and carefully treated, may terminate in a troublesome case of quittor and incurable lameness.
With the passing of colthood, and the coming into effect of the evils of further domestication, the troubles to which the foot is open become more numerous. Foremost among them will come those having their starting-point in errors of practice originating in the forge; for, in spite of attempts at their education, smiths, as a class, are as yet grievously unversed in even the elementary knowledge of the delicate construction of the member that is entrusted to their care.
This fact has been dilated on in books devoted to shoeing, and in the prefatory note to the last edition of Fleming's manual on this subject we find the following statement: 'The records of all humane societies show that, of prosecutions for cruelty to animals, an overwhelming majority refer to the horse; and of these, a large proportion are for working horses while suffering from lameness in one form or other.
'So frequent are such cases that observers have concluded that their prevalence must result from some specific cause, and, not unnaturally, attention has thus been directed to the various modes of management practised in relation to the horse's foot, to the manner of shoeing, and, in particular, to the way in which the foot is prepared for the shoe.'
It must be remembered, however, that although harm in the forge may frequently arise from culpable roughness or carelessness, such is not necessarily always the case, and that quite as much injury may result from careful and conscientious workmanship when it is unfortunate enough to be based upon principles wrong in themselves to commence with.
It so happens, too, that shoeing, in itself a necessary evil, may be responsible for injuries in the causation of which the smith can have played no part. Take, for example, the ill effects following upon the animal's attendant allowing him to carry his shoes for too long a time. In this case the natural growth of the horn carries the heel of the shoe further beneath the foot than is safe for a correct bearing; in fact, anterior to the point of inflection of the wall. The shoe, at the same time, is greatly thinned from excessive wear. Result, a sharp and easily-bended piece of iron situate immediately under the seat of corn. Pressure or actual cutting of the sole is bound to occur, and the animal is lamed.
Again, apart from the question of negligence or otherwise on the part of the smith or the animal's attendant, it must be remembered that the nailing on to the foot of a plate of iron is not giving to the animal an easier means of progression. The reverse is the case. In place of the sucker-like face of the natural horn is substituted a smooth, and, with wear, highly-polished surface. Slipping and sliding attempts to gain a foothold become frequent, and strains of the tendons and ligaments follow in their wake.
As, however, this treatise is not intended to deal with the art of shoeing, the reader must be referred to other works for further information. In addition to Fleming's, there may be mentioned, among others, Hunting's 'Art of Horse Shoeing,' and the very excellent volume of Messrs. Dollar and Wheatley on the same subject. Leaving the forge, we may next look to the nature of the animal's work, and the conditions under which he is kept, for active causes in the production of disorders of the foot. From the yielding softness of the pasture he is called to spend the bulk of his time upon the hard macadamized tracks of our country roads, or the still more hard and more dangerous asphalt pavings or granite sets of our towns. The former, with the bruises they will give the sole and frog from loose and scattered stones, and the latter, with the increased concussion they will entail on the limb, are active factors in the troubles with which we are about to deal. Upon these unyielding surfaces the horse is called to carry slowly or rapidly, as the case may be, not only his own weight, but, in addition, is asked to labour at the hauling of heavy loads. The effects of concussion and heavy traction combined are bound primarily to find the feet, and such diseases as side-bones, ringbones, corns, and sand-cracks commence to make their appearance.
Again, as opposed to the comparative healthiness of the surroundings when at grass, consideration must be given to the chemical changes the foot is frequently subjected to when the animal is housed.
Only too often the bedding the animal has to stand upon for several hours of the twenty-four can only be fitly described as 'filthy in the extreme.' The ammoniacal exhalations from these collected body-discharges must, and do, have a prejudicial effect upon the nature of the horn, and, though slow in its progress, mischief is bound sooner or later to occur in the shape of a weakened and discharging frog, with its concomitant of contracted heels. Lucky it is in such a case if canker does not follow on.
Observers, too, have chronicled the occurrence in horse's feet of disease resulting from the use of moss litter. Tenderness in the foot is first noticeable, which tenderness is afterwards followed by a peculiar softening of the horn of the sole and the frog. What should be a dense, fairly resilient substance is transformed into a material affording a yielding sensation to the fingers not unlike that imparted by a soft indiarubber, and as easily sliced as cheese-rind.
Lastly, though the foot is extremely liable to suffer from the effects of extreme dryness or excessive humidity, especially with regard to the changes thus brought about in the nature of the horn, it is perforce exposed at all times to the varying condition of the roads upon which it must travel. The intense dryness of summer and the constant damp of winter, each in their turn take part in the deteriorating influences at work upon it.
Though this subject might be indefinitely prolonged, this brief resume of the adverse circumstances to which the foot of the horse is exposed is sufficient to point out the extreme importance of its study to the veterinary surgeon. So long as the horse is used as a beast of burden so long will this branch of veterinary surgery offer a wide and remunerative field of labour.
CHAPTER II
REGIONAL ANATOMY
Considered from a zoological standpoint, the foot of the horse will include all those parts from the knee and hock downwards. For the purposes of this treatise, however, the word foot will be used in its more popular sense, and will refer solely to those portions of the digit contained within the hoof. When, in this chapter on regional anatomy, or elsewhere, the descriptive matter or the illustrations exceed that limit, it will be with the object of observing the relationship between the parts we are concerned with and adjoining structures.
Taking the limit we have set, and enumerating the parts within the hoof from within outwards, we find them as follows:
A. THE BONES.—The lower portion of the second phalanx or os coronae; the third phalanx, os pedis, or coffin bone; and the navicular or shuttle bone.
B. THE LIGAMENTS.—The ligaments binding the articulation.
C. THE TENDONS.—The terminal portions of the extensor pedis and the flexor perforans.
D. THE ARTERIES.
E. THE VEINS.
F. THE NERVES.
G. THE COMPLEMENTARY APPARATUS OF THE OS PEDIS.
H. THE KERATOGENOUS MEMBRANE.
I. THE HOOF.
A. THE BONES.
THE SECOND PHALANX, OS CORONAE, OR SMALL PASTERN BONE.—This belongs to the class of small bones, in that it possesses no medullary canal. It is situated obliquely in the digit, running from above downwards and from behind to before, and articulating superiorly with the first phalanx or os suffraginis, and inferiorly with the third phalanx and the navicular bone.
Cubical in shape, it is flattened from before to behind, and may be described as possessing six surfaces: An anterior surface, covered with slight imprints; a posterior surface, provided above with a transversely elongated gliding surface for the passage of the flexor perforans; two lateral surfaces, each rough and perforated by foraminae, and each bearing on its lower portion a thumb-like imprint for ligamentous attachment, and for the insertion of the bifid extremity of the perforatus tendon; a superior surface, bearing two shallow articular cavities, separated by an antero-posterior ridge, for the accommodation of the lower articulating surface of the first phalanx; an inferior surface, also articulatory, which in shape is obverse to the superior, bearing two unequal condyles, separated by an ill-defined antero-posterior groove, which surface articulates with the os pedis and the navicular bone.
Development.—The bone usually ossifies from one centre, but often there is a complementary nucleus for the upper surface.
THE THIRD PHALANX, OS PEDIS, OR COFFIN BONE.—This also belongs to the class of short bones. It forms the termination of the digit, and, with the navicular bone, is included entirely within the hoof. For our examination it offers three surfaces, two lateral angles, and three edges.
The Anterior or Laminal Surface, following closely in contour the wall of the hoof, is markedly convex from side to side, nearly straight from above to below, and closely dotted with foraminae of varying sizes. On each side of this surface is to be seen a distinct groove, the preplantar groove, or preplantar fissure, which, commencing behind, between the basilar and retrossal processes, runs horizontally forwards from the angles or wings of the bone, and terminates anteriorly in one of the larger foraminae. As the name 'laminal' indicates, it is this surface which in the fresh state is covered by the sensitive laminae.
The Inferior or Plantar Surface, hollowed in the form of a low arch, presents for our inspection two regions, an anterior and a posterior, divided by a well-marked line, the Semilunar Crest, which extends forward in the shape of a semicircle. The anterior region, as is the laminal surface, is covered with foraminae; in this case more minute. In the recent state it is covered by the sensitive sole. The posterior region, lying immediately behind the semilunar crest, shows on each side of a median process a large foramen, the Plantar Foramen. From this foramen runs the Plantar Groove, a channel, bounded above by the superior edge, and below by the semilunar crest of the bone, which conducts the plantar arteries into the Semilunar Sinus, a well-marked cavity in the interior of the bone.
The Superior or Articular Surface consists of two shallow depressions, divided by a slight median ridge. Its posterior part shows a transversely elongated facet for articulation with the navicular bone.
The Superior Edge, outlining the superior margin of the laminal surface, describes a curve, with the convexity of the curve forward. In the centre of the curve is a triangular process, the Pyramidal Process, which serves as the point of attachment of the extensor pedis.
The Inferior Edge, the most extensive of the three, separates the laminal from the solar surface. It is semicircular in shape, sharp, and finely dentated, and is perforated by eight to ten large foraminae.
The Posterior Edge, very slightly concave, divides the small, transversely elongated facet of the superior surface from the posterior region of the inferior surface.
The Lateral Angles of the bone, also termed the Wings, are two projections directed backwards. Each is divided by a cleft into an upper, the Basilar Process, and a lower, the Retrossal Process. In old animals the posterior portion of the cleft separating the two processes gradually becomes filled in with bony deposit, thus transforming the cleft into a foramen, which gives passage to the preplantar artery. We may mention in passing that the lateral angles give attachment to the lateral fibro-cartilages, and that the lateral angles themselves in old horses become increased in size owing to ossification of portions of the adjacent lateral cartilages.
Development.—The os pedis ossifies from two centres, one of which is for the articular surface; but this epiphysis fuses with the rest of the bone before birth.
THE NAVICULAR BONE, SHUTTLE BONE, OR SMALL SESAMOID.—Placed behind the articulating point of the second and third phalanges, this small shuttle-shaped bone assists in the formation of the pedal articulation. It is elongated transversely, flattened from above to below, and narrow at its extremities. In it we see two surfaces, and two borders.
The Superior or Articular Surface of the bone, which may easily be recognised by its smoothness, is moulded upon the lower articular surface of the second phalanx, being convex in its middle, and concave on either side.
The Inferior or Tendinous Surface resembles the preceding in form, but is broader and less smooth. In the recent state it is covered with fibro-cartilage for the passage of the flexor perforans.
The Anterior Border possesses above a small transversely elongated facet for articulation with the os pedis, and below a more extensive grooved portion, perforated by numerous foraminae, affording attachment to the interosseous ligaments of the articulation.
The Posterior Border, thick in the middle, but thinner towards the extremities, is roughened for ligamentous attachment.
Development.—The bone ossifies from a single centre.
B. THE LIGAMENTS.
THE ARTICULATION OF THE FIRST WITH THE SECOND PHALANX, OR THE PASTERN JOINT.—Adhering to the limit we have set, this articulation should not receive our attention. As, however, we shall in a later page be concerned with fractures of the os coronae, which fractures may affect the articulation above mentioned, a brief note of its formation will not be out of place.
It is an imperfect hinge-joint, permitting of extension and flexion, allowing the first phalanx to pivot on the second, and admitting of the performance of slight lateral movements. It is formed by the opposing of the inferior surface of the os suffraginis with the superior surface of the os coronae. The articulating surface of the os coronae is supplemented by the addition behind of a thick piece of fibro-cartilage (the glenoid) attached inferiorly to the posterior edge of the upper articulatory surface of the os coronae, and superiorly by means of three fibrous slips on each side to the os suffraginis. The innermost of these three slips becomes attached to about the middle of the lateral edge of the suffraginis, and the remaining two, beneath the first, attach themselves to nearer the lower end of that bone. The posterior surface of the complementary cartilage forms a gliding surface for the passage of the perforans.
The Lateral Ligaments.—These are large and thick, an outer and an inner, running obliquely from above downwards and backwards. Each is inserted superiorly into the lateral tubercle of the lower end of the first phalanx, and inferiorly to the side of the second phalanx, their most inferior fibres becoming finally fixed to the extremities of the navicular bone, where they form the postero-lateral ligaments of the pedal articulation. In front of the joint the extensor pedis plays the part of an additional ligament.
The Synovial Membrane.—This is limited in front by the tendon of the extensor pedis, on each side by the lateral ligaments of the joint, and behind by the glenoid fibro-cartilage. At this point it is prolonged upwards as a pouch behind the lower extremity of the first phalanx.
THE ARTICULATION OF THE SECOND PHALANX WITH THE THIRD, THE PEDAL, OR THE COFFIN JOINT.—This also is an imperfect hinge-joint, permitting only of flexion and extension, which movements are more restricted than in the previous articulation. Three bones enter into its formation: the second phalanx, the third phalanx, and the navicular bone. The lower articulatory surface is formed by the third phalanx and the navicular bone combined. To effect this the navicular is closely and firmly attached to the third phalanx by an interosseous ligament. The two bones, as one, are then connected to the second phalanx by four lateral ligaments, an anterior and a posterior on each side.
The Interosseous Ligament consists of extremely short fibres running from the extensively grooved portion of the anterior surface of the navicular bone to become attached to the os pedis immediately behind its articular surface.
The Antero-lateral Ligaments are attached by their superior extremities to the lateral surfaces of the second phalanx, and by their inferior extremities into the depressions on either side of the pyramidal process of the os pedis.
The Postero-lateral Ligaments.—As mentioned when describing the first interphalangeal articulation, these are in reality continuations of the lateral ligaments of that joint. Running obliquely downwards and backwards from their point of attachment to the first phalanx they curve round the lower part of the side of the second phalanx and end on the extremities and posterior surface of the navicular bone. Having reached that position, they send short attachments to the retrossal process of the os pedis and to the inner face of the lateral cartilage.
Synovial Membrane.—This extends below the facets uniting the navicular to the pedal bone, and offers for consideration two sacs. A large one posteriorly running up behind the second phalanx to nearly adjoin the sesamoidean bursae, and a small one, a prolongation of the synovial membrane between the antero-lateral and postero-lateral ligaments of the same side. This latter is often distended, and on account of its close proximity to the seat of operation, is liable to be accidentally opened in excision of the lateral cartilage for quittor.
C. THE TENDONS
In order to convey an intelligent understanding of the tendons it will be wise to briefly describe the course of their parent muscles from their commencement.
THE EXTENSOR PEDIS.—The extensor pedis arises from the lower extremity of the humerus in two distinct portions of unequal size, a muscular and a tendinous. These are succeeded by two tendons passing in common through a vertical groove at the lower end of the radius. Lower in the limb these tendons separate, the outer and smaller joining the tendon of the extensor suffraginis, and the inner and main tendon continuing its course downwards. With the exception of the navicular, it is attached to all the bones of the foot, and is covered internally by the capsular ligaments of the joints over which it passes, those with which we are concerned being the pastern joint and the pedal joint. Before its attachment to the os pedis it receives on each side of the middle of the first phalanx reinforcement in the shape of a strong band descending obliquely over the fetlock from the suspensory ligament. Widening out in fanlike fashion, it is inserted into the pyramidal process of the os pedis.
Action.—The action of this muscle is to extend the third phalanx on the second, the second on the first, and the first on the metacarpus. It also assists in the extension of the foot on the forearm.
THE FLEXOR PEDIS PERFORATUS, OR THE SUPERFICIAL FLEXOR OF THE PHALANGES.—In common with the perforans, this muscle arises from the inner condyloid ridge of the humerus. It is reinforced at the lower end of the radius by the superior carpal ligament, passes through the carpal and metacarpo-phalangeal sheaths, and, arriving behind the fetlock, forms a ring for the passage of the flexor perforans. Its termination is bifid, and it is inserted on either side to the lateral surface of the second phalanx.
[Illustration: FIG. 12.—THE FLEXOR PERFORATUS AND FLEXOR PERFORANS TENDONS. The metacarpo-phalangeal sheath and the ring of the perforatus laid open posteriorly, and the cut edges reflected; the flexor perforans cut through at about the region of the sesamoids, and its inferior portion deflected. 1, Superior end of severed perforans tendon; 2, inferior end of severed perforans tendon; 3, insertion of flexor perforans into semilunar crest of os pedis; 4, the cut and reflected edges of the metacarpo-phalangeal sheath and perforatus ring; 5, the bifid insertion of the flexor perforatus into the lateral surfaces of the os corona; 6, the capsular ligament of the pedal joint; 7, the navicular bone; 8, the posterior surface and glenoid fibro-cartilage of the os coronae.]
Action.—This muscle flexes the second phalanx on the first, the first on the metacarpus, and the entire foot on the forearm. Mechanically, it acts as a stay when the animal is standing by maintaining the metacarpo-phalangeal angle.
THE FLEXOR PEDIS PERFORANS, OR THE DEEP FLEXOR OF THE PHALANGES.—This muscle consists of three easily-divided portions: an ulnar, a humeral, and a radial, and has for points of origin the olecranon process of the ulna, the inner condyloid ridge of the humerus, and the posterior surface of the radius. These portions are continued by a common tendon which enters the carpal sheath with the tendon of the perforatus, and continues with it through the synovial sheath of the metacarpo-phalangeal region. Like the last-named tendon, it receives a supporting check-band, in this case from the posterior ligament of the carpus. Passing down between the suspensory ligament in front, and the perforatus tendon behind, it glides over the sesamoid pulley and passes through the ring formed by the perforatus. Continuing its course, it passes between the bifurcating portions of the extremity of the perforatus, glides over the smooth posterior surface of the supplementary glenoid cartilage of the articulation of the first and second phalanges, plays over the inferior surface of the navicular bone, and finally becomes inserted into the semilunar crest of the os pedis. On reaching the posterior border of the navicular bone it widens out to form the plantar aponeurosis.
In connection with the lower portion of this tendon must be noticed the Navicular Sheath. This is a synovial sheath lining the deep face of the tendon, and reflected on to the navicular bone and the interosseous ligament of the pedal joint. This will be of particular interest when we come to deal with cases of pricked foot from picked up nails. Above, it is in connection with the synovial membrane of the pedal articulation and that of the metacarpo-phalangeal sheath.
Action.—The action of the perforans is to flex the third on the second, and the second on the first phalanx. The latter it flexes in turn on the metacarpus. It also assists in the flexion of the entire foot on the forearm, and in supporting the angle of the metacarpo-phalangeal articulation when the animal is standing.
D. THE ARTERIES.
So far as the arteries supplying the foot are concerned, we shall be interested in following up the distribution of the two digitals, which are the terminal branches of the Large Metacarpal.
THE LARGE METACARPAL, OR COLLATERAL ARTERY OF THE CANNON.—This, the larger terminal branch of the posterior radial artery, needs brief mention, for the reason that we shall be afterwards concerned with it in the operation of neurectomy. Its point of origin is the inside of the inferior extremity of the radius. Descending in company with the flexor tendons, and passing behind the carpus and beneath the carpal sheath, it continues its descent, in company with the internal plantar nerve and the internal metacarpal vein, on the inner side of the flexor tendons until just above the fetlock. At this point it bifurcates into the digital arteries.
From the carpus downwards the large metacarpal artery, the internal metacarpal vein, and the internal plantar nerve are in close relation with each other. The vein holds the anterior position. The artery is between the two, and has the nerve in close contact with it behind.
THE DIGITAL ARTERIES, OR COLLATERAL ARTERIES OF THE DIGIT.—These are of large volume, and carry the blood to the keratogenous apparatus of the foot. They separate from each other at an acute angle, and pass over the side of the fetlock, one to the inside, the other to the outside, to reach the internal face of the basilar process of the os pedis, where they bifurcate to form the Plantar and Preplantar arteries. In the whole of their course the digital arteries follow the flexor tendons, and are related in front to the digital vein, and behind to the posterior branch of the plantar nerve. This is the nerve implicated in the lower operation of neurectomy, and its relation to adjoining structures will be detailed under Section F. of this chapter. During its course the digital artery gives off branches in the following positions:
1. At the Fetlock numerous branches to the metacarpo-phalangeal articulation, the sesamoid sheath, and the tendons.
2. At the Upper Extremity of the First Phalanx branches for the supply of the surrounding tissues, and for the tissues of the ergot.
3. Towards the Middle of the Third Phalanx, the Perpendicular artery of Percival. This arises at a right angle from the main vessel, and immediately divides into two series of ramifications—an ascending and a descending. The ramifications of these series freely anastomose with corresponding vessels of the opposite side.
4. At the Superior Border of the Lateral Cartilage, the Artery of the Plantar Cushion. This is directed obliquely downwards and backwards, under cover of the cartilage, and is distributed to the middle portion of the complementary apparatus of the os pedis, as well as to the villous tissue and the coronet. A branch of it is turned forwards to join with the coronary circle in forming the circumflex artery of the coronet.
5. Under the Lateral Cartilage two transverse branches, an anterior and a posterior, to form the Coronary Circle. The numerous ramifications of these branches anastomose both anteriorly and posteriorly with their corresponding branches of the artery of the opposite side. This circle closely embraces the os coronae. Among the larger branches given off from its anterior portion are two descending, one on each side of the extensor pedis, to assist in the formation of the Circumflex Artery of the Coronary Cushion. The formation of this last-named artery is completed posteriorly by the before-mentioned branch from the artery of the plantar cushion.
THE PREPLANTAR (UNGUAL[A]) ARTERY.—This, the smaller of the two terminal branches of the digital, is situated inside the basilar process of the os pedis. It turns round this to gain the fissure between the basilar and retrossal processes, and becomes lodged in the preplantar fissure. Here it terminates in several divisions which bury themselves in the os pedis. Before leaving the inner aspect of the pedal wing it supplies a deep branch to the heel and the villous tissue. Gaining the outer aspect of the wing, it distributes a further backward branch, which passes behind the circumflex artery of the pedal bone, and, during its passage in the preplantar fissure, gives off ascending and descending branches, which ramify in the laminal tissue.
THE PLANTAR (UNGUAL[A]) ARTERY.—This, the larger of the two terminals of the digital, may be looked upon as a continuation of the main vessel. Running along the plantar groove, it gains the plantar foramen. Here it enters the interior of the bone (the semilunar sinus) and anastomoses with the corresponding artery of the opposite side. The circle of vessels so formed is called the Plantar Arch or the Semilunar Anastomosis.
[Footnote A: The epithet 'ungual' is added by Chauveau to distinguish these arteries from the properly so-called plantar arteries—the terminal divisions of the posterior tibial artery.]
From the semilunar anastomosis radiate two main groups of arterial branches, an ascending group and a descending one. The ascending branches penetrate the substance of the os pedis, and emerge by the numerous foraminae on its laminal surface. The descending branches, larger in size, also penetrate the substance of the pedal bone, and emerge in turn from the foraminae cribbling its outer surface—in this case the set of larger foraminae opening on its inferior edge. Having gained exit from the bone, their frequent anastomosis, right and left, with their fellows forms a large vessel following the contour of the inferior edge of the os pedis. This constitutes the Circumflex Artery of the Toe.
E. THE VEINS.
These commence at the foot with a series of plexuses, which may be described as forming (1) AN INTERNAL OR INTRA-OSSEOUS VENOUS SYSTEM, and (2) AN EXTERNAL OR EXTRA-OSSEOUS VENOUS SYSTEM.
1. THE INTRA-OSSEOUS VENOUS SYSTEM.—This is a venous system within the structure of, and occupying the semilunar sinus of the os pedis. It follows in every respect the arrangement of the arteries as before described in the same region. Efferent vessels emerge from the plantar foraminae, follow the plantar fissures, and ascend within the basilar processes of the os pedis. Here they lie under shelter of the lateral cartilages, and assist in the formation of the deep layer of the coronary plexus of the extra-osseous system.
2. THE EXTRA-OSSEOUS VENOUS SYSTEM.—This may be regarded as a close-meshed network enveloping the whole of the foot. Although a continuous system, it is best described by recognising in it three distinct parts:
(a) The Solar Plexus. (b) The Podophyllous Plexus. (c) The Coronary Plexus.
(a) The Solar Plexus.—The veins of this plexus discharge themselves in two directions: (1) By a central canal or canals running along the bottom of the lateral lacunae of the plantar cushion to gain the deep layer of the coronary plexus. (2) By the Circumflex or Peripheral Vein of the Toe, a canal formed by ramifications from the solar and the podophyllous plexuses, and following the direction of the artery of the same name. The circumflex vein terminates by forwarding branches to concur in the formation of the superficial coronary plexus.
(b) The Podophyllous or Laminal Plexus.—The podophyllous veins anastomose below with the circumflex vein of the solar plexus, and above with the veins of the coronary plexus.
(c) The Coronary Plexus.—This proceeds from the podophyllous, the intra-osseous, and the solar networks, and consists of a central and two lateral parts.
The central portion lies between the lateral cartilages and immediately under the coronary cushion. The lateral portions are ramifications on both surfaces of the lateral cartilages. The ramifications on the lateral cartilages may be again distinguished as superficial and deep. The superficial layer is distributed over the external face of the cartilage, forming thereon a dense network, and finally converges towards the superior limit of the plexus to form ten or twelve principal branches, which again unite to form two large vessels. These vessels, by their final fusion at the lower end of the first phalanx, constitute the digital vein. The deep layer is formed, as before described, by ascending branches from the posterior parts of the podophyllous and solar plexuses, and by branches from the intra-osseous system of the pedal bone. The veins of this deep layer finally drain into the two vessels proceeding from the superficial layer, which go to the formation of the digital vein.
THE DIGITAL VEINS—These arise from the network formed on the surfaces of the lateral cartilages, and ascend in front of the digital arteries to unite above the fetlock, where they form an arch between the deep flexor and the suspensory ligament. From this arch (named the Sesamoidean) proceed the Metacarpal Veins.
THE METACARPAL VEINS.—Three in number, they are distinguished as an Internal and an External Metacarpal, and a Deep or Interosseous Metacarpal. As we shall be concerned with these in the higher operation of neurectomy, we may give them brief mention.
THE INTERNAL METACARPAL VEIN, the largest of the three, has relations with the internal metacarpal artery and the internal plantar nerve. These relations were shortly discussed under the section devoted to the arteries, to which the reader may refer.
THE EXTERNAL METACARPAL VEIN.—This ascends on the external side of the flexor tendons in company with the external plantar nerve.
The Interosseous Vein.—This is an irregular vessel running up between the suspensory ligament and the posterior face of the large metacarpal bone.
F. THE NERVES.
THE PLANTAR NERVES.—These are two in number, and are distinguished as Internal and External.
THE INTERNAL PLANTAR NERVE lies behind and in close contact with the great metacarpal artery during that vessel's course down the region of the cannon. A point of interest is that it gives off at about the middle of the cannon a branch which bends obliquely downwards and behind the flexor tendons to join its fellow of the opposite side—namely, the external plantar. This it joins an inch or more above the bottom of the splint bone. Measured in a straight line, this is about 2-1/2 inches below its point of origin. Near the fetlock, at the level of the sesamoids, the internal plantar nerve ends in several digital branches.
THE EXTERNAL PLANTAR NERVE.—This holds a position to the outside of the metacarpal region, analogous to that of the internal plantar nerve on the inside of the limb, running down on the external edge of the flexor tendons. Unlike the internal nerve, it is accompanied by a single vessel only, the external metacarpal vein, behind which it lies. At the level of the sesamoid bones it divides, as does the internal nerve, into three main branches—the digital nerves.
THE DIGITAL NERVES.—These are distinguished as Anterior, Middle, and Posterior.
The Anterior Branch descends in front of the vein, distributing cutaneous branches to the front of the digit, and terminating in the coronary cushion.
The Middle Branch descends between the artery and the vein, and freely anastomoses with the two other branches. It terminates in the coronary cushion and the sensitive laminae.
The Posterior Branch.—This is the largest of the three, and may be regarded as the direct continuation of the plantar. At the fetlock it is placed immediately above the digital artery, but afterwards takes up a position directly behind that vessel. Together with the digital artery it descends to near the basilar process of the os pedis. Here it passes with the plantar artery into the interior of the os pedis, and continues its main branch, with the preplantar artery, in the fissure of the same name, to finally furnish supply to the os pedis and the sensitive laminae. It is this nerve which is divided in the low operation of neurectomy.
Beyond the fact of this branch descending, in the region of the pastern, 1 inch behind the digital artery, a further point of interest presents itself to the surgeon, and one to which attention must be paid. This is the presence in close proximity to the nerve of the Ligament of the Pad (Percival), or the Ligament of the Ergot (McFadyean). This is a subcutaneous glistening cord originating in the ergot of the fetlock, passing in an oblique direction downwards and forwards, and crossing over on its way both the digital artery and the posterior branch of the digital nerve.
In the foregoing description of the anatomy, we have taken the fore-limb as our guide. In the hind-limb, where they reach the foot, the counterparts of the tendons, arteries, veins, and nerves differ in no great essential from their fellows in the fore. They will therefore need no special mention.
G. THE COMPLEMENTARY APPARATUS OF THE OS PEDIS.
This consists of two lateral pieces, the LATERAL CARTILAGES or Fibro-cartilages of the pedal bone, united behind and below by the Plantar Cushion.
1. THE LATERAL CARTILAGES.—Each is a flattened plate of cartilage, possessing two faces and four borders separated by four angles.
The external face is convex, covered by a plexus of veins, and slightly overhangs the pedal bone. The internal face is concave, and covers in front the pedal articulation and the synovial sac, already mentioned as protruding between the antero- and postero-lateral ligaments of that joint. We have already remarked that this is a point of interest to be remembered in connection with the operation for quittor. Below and behind, the internal face of the cartilage is united to the plantar cushion.
The upper border, sometimes convex, sometimes straight, is thin and bevelled, and may easily be felt in the living animal. It is this border that the digital vessels cross to gain the foot, and the border is often broken by a deep notch to accommodate them. The inferior border is attached in front to the basilar and retrossal processes, behind which it blends with the plantar cushion. The posterior border is oblique from before to behind, and above to below, and joins the preceding two. The anterior border is oblique in the same direction, and is intimately attached to the antero-lateral ligament of the pedal articulation. The cartilages of the fore-feet are thicker and more extensive than those of the hind.
2. THE PLANTAR CUSHION on FIBRO-FATTY FROG.—Composed of a fibrous meshwork, in the interstices of which are lodged fine elastic and connective fibres and fat cells, this wedge-shaped body occupies the space between the two lateral cartilages, the extremity of the perforans tendon, and the horny frog. It offers for consideration an antero-superior and an infero-posterior face, a base, an apex, and two borders.
The antero-superior face is in contact with the terminal expansion of the perforans tendon. The infero-posterior face is covered by the keratogenous membrane, and follows closely the shape of the horny frog, on whose inner surface it is moulded. It presents, therefore, at its centre a single conical prolongation, the Pyramidal Body, which is continued behind, as is the horny frog, in the shape of two lateral ridges divided by a median cleft. The base of the cushion lies behind, and consists of two lateral masses, the Bulbs of the Plantar Cushion. In front these are continuous with the ridges of the pyramidal body, while behind they become confounded with the lateral cartilages and the coronary cushion. The apex is fixed into the plantar surface of the os pedis, in front of its semilunar ridge. The borders, right and left, are wider behind than before, and are in relation with the inner faces of the lateral cartilages.
H. THE KERATOGENOUS MEMBRANE.
THE KERATOGENOUS, OR HORN-PRODUCING MEMBRANE, is in reality an extension of the dermis of the digit. It covers the extremity of the digit as a sock covers the foot, spreading over the insertion of the extensor pedis, the lower half of the external face of the lateral cartilages, the bulbs of the plantar cushion, the pyramidal body, the anterior portion of the plantar surface of the os pedis, and over the anterior face of the same bone. In turn, as the human foot with its sock is covered by the boot, this is encased by the hoof, the formation of which we shall study later.
To expose the membrane for study the hoof must be removed. This may be done in two ways. By roasting in a fire, and afterwards dragging off the horny structures with a pair of pincers, a knife having first been passed round the superior edge of the horny box. Or by maceration in water for several days, when the hoof will become loosened by the process of decomposition, and may be easily removed by the hands. The latter method is less likely to injure the sensitive structures, and will expose them with a fresh appearance for observation.
For purposes of description the keratogenous membrane is divided into three regions:
1. The Coronary Cushion. 2. The Velvety Tissue. 3. The Podophyllous Tissue, or the Sensitive laminae.
1. THE CORONARY CUSHION. In the foot stripped of the hoof the coronary cushion is seen as a rounded structure overhanging the sensitive laminae after the manner of a cornice. It extends from the inner to the outer bulbs of the plantar cushion, and is bounded above by the perioplic ring, and below by the laminae.
When in situ it is accommodated by the Cutigeral Groove, a cavity produced by the bevelling out of the superior portion of the inner face of the wall of the hoof. Its superior surface is covered by numerous elongated papillae, set so closely as to give the appearance of the 'pile' of velvet. This is observed to the best advantage with the foot immersed in water.
The Superior Border of the cushion is bounded by the Perioplic Ring, the cells of which have as their function the secreting of the Periople, a layer of thin horn to be noted afterwards as covering the external face of the wall. From the perioplic ring the cushion is separated by a narrow and shallow, though well-marked, groove.
The inferior border is bounded by the sensitive laminae.
The upper portions of the laminae, those in contact with the cushion, are pale in contrast with the portions immediately below, and thus there is given the appearance of a white zone adjoining the inferior border of the cushion.
Widest at its centre, the cushion narrows towards its extremities, which, arriving at the bulbs of the plantar cushion, bend downwards into the lateral lacunae of the pyramidal body, where they merge into the velvety tissue of the sole and frog.
The papillae of the coronary cushion secrete the horn tubules forming the wall, and the papillae of the perioplic ring secrete the varnish-like veneer of thin horn covering the outside surface of the hoof.
—(a) its median lacuna, (6) its lateral lacuna; 3. V-shaped depression accommodating the toe-stay; 4, the sensitive laminae which interleave with the horny laminae of the bar.]
[Footnote A: The sensitive frog thinly invests the plantar cushion or fibre-fatty frog, the outline of which is here indicated.]
2. THE VELVETY TISSUE.—This is the portion of the keratogenous membrane covering the plantar surface of the os pedis and the plantar cushion. To the irregularities of the latter body—its bulbs, pyramidal body, and its lacunae—it is closely adapted. Its surface may, therefore, be divided into (a) The Sensitive Frog, and (b) The Sensitive Sole.
(a) The Sensitive Frog is that part of the velvety tissue moulded on the lower surface of the plantar cushion. The shape of the plantar cushion has already been described as identical with that of the horny frog. It only remains to state that, like the coronary cushion, the surface of the sensitive frog is closely studded with papillae. The cells clothing the papillae are instrumental in forming the horny frog.
(b) The Sensitive Sole.—As its name indicates, this is the portion of the keratogenous membrane that covers the plantar surface of the os pedis. It also is clothed with papillae, which again give rise to the formation of that part of the horny box to which they are adapted—namely, the sole.
3. THE PODOPHYLLOUS TISSUE, OR SENSITIVE LAMINAE.—This portion of the keratogenous membrane is spread over the anterior face and sides of the os pedis, limited above by the coronary cushion, and below by the inferior edge of the bone. It presents the appearance of fine longitudinal streaks, which, when closely examined with a needle, are found to consist of numerous fine leaves. These extend downwards from the lower border of the coronary cushion to the inferior margin of the os pedis. At this point each terminates in several large villous prolongations, which extend into the horny tubes at the circumference of the sole. At the point of the toe this membrane sometimes shows a V-shaped depression, into which fits a inverted V-shaped prominence on the inner surface of the wall at this point.
The sensitive laminae increase in width from above to below. Their free margin is finely denticulated, while their sides are traversed from top to bottom by several folds (about sixty), which, examined microscopically, are seen to consist of secondary leaves, or laminellae.
Examined on the foot, deprived of its horny covering, the sensitive laminae are, the majority of them, in close contact with each other. In the normal state this is not so. The interstices between the leaves are then occupied by the horny leaves, to be afterwards described as existing on the inner surface of the wall.
Reaching and rounding the heels, the sensitive laminae extend forward for a short distance, where they interleave with the horny laminae of the bars.
Much discussion has centred round the point as to whether or no the cells of the sensitive laminae take any share in the formation of the horn of the wall. This will be alluded to in a future chapter.
I. THE HOOF.
Removed from the foot by maceration a well-shaped hoof is cylindro-conical in form, and appears to the ordinary observer to consist of a box or case cast in one single piece of horn. Prolonged maceration, however, will show that the apparently single piece is divisible into three. These are known as (1) THE WALL, (2) THE SOLE, and (3) THE FROG. In addition to these, we have also an appendage or circular continuation of the frog named (4) THE PERIOPLE, or CORONARY FROG BAND. These various divisions we will study separately.
1. THE WALL is that portion of the hoof seen in front and laterally when the horse's foot is on the ground. Posteriorly, instead of being continued round the heels to complete the circle, its extremities become suddenly inflected downwards, forwards, and inwards. These inflections can only be seen with the foot lifted from the floor, and form the so-called Bars. It will be noticed, too, with the foot lifted, that the wall projects beyond the level of the other structures of the plantar surface, taking upon itself the bearing of the greatest part of the animal's weight.
The horn of the wall, viewed immediately from the front, is known as the Toe, which again is distinguished as Outside Toe or Inside Toe, according as the horn to its inner or outer aspect is indicated. The remainder of the external face of the wall, that running back to the heels, is designated the Quarters.
In the middle region of the toe, the wall following the angle of the bones is greatly oblique. This obliquity decreases as the quarters are reached, until on reaching the heels the wall is nearly upright.
For observation the wall offers two faces, two borders, and two extremities.
The External Face is convex from side to side, but straight from the upper to the lower border. Examined closely, it is seen to be made up of closely-arranged parallel fibres running in a straight line from the upper to the lower border, and giving the surface of the foot a finely striated appearance. In addition to these lines, which are really the horn tubules, the external face is marked by a series of rings which run horizontally from heel to heel. These are due to varying influences of food, climate, and slight or severe disease. This will be noted again in a later page. In a young and healthy horse the whole of the external face of the wall is smooth and shining. This appearance is due to a thin layer of horn, secreted independently of the wall proper, termed the periople.
The Internal Face of the wall, that adapted to the sensitive laminae, is closely covered over its entire surface with white parallel leaves (Keraphyllae, or horn leaves, to distinguish them from the Podophyllae, or sensitive leaves). These keraphyllae dovetail intimately with the sensitive laminae, covering the os pedis. Running along the superior portion of the inner face is the Cutigeral Groove. This cavity has been mentioned before as accommodating the coronary cushion, whose shape and general contour it closely follows, being widest and deepest in front, and gradually decreasing as it proceeds backwards. It is hollowed out at the expense of the wall, and shows on its surface numberless minute openings which receive the papillae of the coronary cushion.
At the bottom of the internal face, at the point where the toe joins the sole, will be noted the before-mentioned inverted V-shaped prominence. Its position will be clearly understood when we say that it gives the appearance of having been forced there by the pressure of the toe-clip of the shoe. This will be noted again when dealing with the sole.
The Inferior Border of the wall offers little to note. It is that portion in contact with the ground, and subject to wear. A point of interest is its union with the sole. This will be noticed in a foot which has just been pared as a narrow white or faint yellow line on the inner or concave face of the wall at its lower portion. It marks the point where the horny leaves of the wall terminate and become locked with corresponding leaves of the circumference of the sole.
The Superior Border follows closely the line marked by the perioplic ring and the groove separating the latter from the coronary cushion.
The Extremities of the wall are formed by the abruptly reflected portions of the wall at the heels. Termed by some the 'Inflexural Nodes,' they are better known to us as the 'Points of the Heels.'
2. THE SOLE.—The sole is a thick plate of horn which, in conjunction with the bars and the frog, forms the floor of the foot. In shape it is irregularly crescentic, its posterior portion, that between the horns of the crescent, being deeply indented in a V-shaped manner to receive the frog. Its upper surface is convex, its lower concave. It may be recognised as possessing two faces and two borders.
The Superior or Internal Face is adapted to the sole of the os pedis. Its highest point, therefore, is at the point of its V-shaped indentation. From this point it slopes in every direction downwards and outwards until near the circumference. Here it curves up to form a kind of a groove in which is lodged the inferior edge of the os pedis. In the centre of its anterior portion—that is to say, at the toe—will be seen a small inverted V-shaped ridge, which is a direct continuation of the same shaped prominence before mentioned on the internal face of the wall. This Fleming has termed the toe-stay, from a notion that it serves to maintain the position of the os pedis. The whole of the superior face of the sole is covered with numerous fine punctures which receive the papillae of the sensitive sole.
The Inferior Face is more or less concave according to circumstances, its deepest part being at the point of the frog. Sloping from this point to its circumference, it becomes suddenly flat just before joining the wall. Its horn in appearance is flaky.
The External Border or Circumference is intimately dovetailed with the horny laminae of the wall. At its circumference the sole, if unpared, is ordinarily as thick as the wall. This thickness is maintained for a short distance towards its centre, after which it becomes gradually more thin.
The Internal Border has the shape of an elongated V with the apex pointing forwards. It is much thinner than the external border, and, like it, is dovetailed into the horny laminae of the inflections of the wall—namely, the bars. In front of the termination of the bars it is dovetailed into the sides and point of the frog. Where unworn by contact with the ground, the horn of the sole is shed by a process of exfoliation.
3. THE FROG.—Triangular or pyramidal in shape, the frog bears a close resemblance to the form of the plantar cushion, upon the lower surface of which body it is moulded. It offers for consideration two faces, two sides, a base, and a point or summit.
The Superior Face is an exact cast of the lower surface of the plantar cushion. It shows in the centre, therefore, a triangular depression, with the base of the triangle directed backwards. Posteriorly, the depression is continued as two lateral channels divided by a median ridge. The median ridge widens out as it passes backwards, forming the larger part of the posterior portion of the frog. This median ridge fits into the cleft of the plantar cushion. It serves to prevent displacement of the sensitive from the horny frog, and has been rather aptly termed the 'Frog-stay.'
The Inferior Surface is an exact reverse of the superior. The triangular depression of the superior surface is represented in the inferior surface by a triangular projection, and the ridge-like frog-stay of the upper surface is represented below by a median cleft, the Median Lacuna of the frog. The triangular projection in front of the median lacuna is the body or cushion of the frog. It is continued backwards as two ridge-like branches, which, at the points of the heels, form acute angles with the bars. On the outer side of each lateral ridge is a fissure. These are known as the Lateral Lacunae.
The Sides of the frog are flat and slightly oblique. They are closely united to the bars and to the triangular indentation in the posterior border of the sole.
The Base of the frog is formed by the extremities of its branches, which, becoming wider and more convex as they pass backwards, form two rounded, flexible, and elastic masses separated from each other by the median lacuna. These constitute the 'glomes' of the frog. They are continuous with the periople.
The Point of the Frog is situated, wedge-like, within the triangular notch in the posterior border of the sole.
4. THE PERIOPLE, OR CORONARY FROG BAND.—This is a continuation of the substance of the frog around the extreme upper surface of the hoof. It is widest at the heels over the bulbs or glomes of the frog, and gradually narrows as it reaches the front of the hoof. It is, in reality, a thin pellicle of semi-transparent horn secreted by the cells of the perioplic ring. When left untouched by the farrier's rasp it serves the purpose, by acting as a natural varnish, of protecting the horn of the wall from the effects of undue heat or moisture.
CHAPTER III
GENERAL PHYSIOLOGICAL AND ANATOMICAL OBSERVATIONS
The matter embraced by the heading of this chapter will offer for discussion many subjects of great interest to the veterinary surgeon. Around some of them debate has for many years waxed more than keen. Of the points in dispute, some of them may be regarded as satisfactorily settled, while others offer still further room for investigation.
In this volume we can only hope to deal with them in brief, and must select such as appear to have the greatest bearing on the veterinarian's everyday practice.
Always prolific of heated discussion has been one question: 'Are the horny laminae secreted by the sensitive?' To answer this satisfactorily, it will be best to give a short account of the mode of production of the hoof in general.
A. DEVELOPMENT OF THE HOOF.
Starting with the statement that it is epidermal in origin, we will first consider the structure of the skin, and follow that with a brief description of the structure and mode of growth of the human nail, a short study of which will greatly assist us when we come to investigate the manner of growth of the horse's hoof.
THE SKIN is composed of two portions, the EPIDERMIS and the CORIUM.
THE EPIDERMIS is a stratified epithelium. The superficial layers of the cells composing it are hard and horny, while the deeper layers are soft and protoplasmic. These latter form the so-called Retae Mucosum of Malpighi.
Commencing from below and proceeding upwards, we find that the lowermost cells of the rete mucosum, those that are set immediately on the corium, are columnar in shape. In animals that have a coloured skin these cells contain pigment granules. Directly superposed to these we find cells which in shape are polyhedral. Above them, and forming the most superficial layer of the rete mucosum, is a series of flattened, granular-looking cells known as the stratum granulosum.
Immediately above the stratum granulosum the horny portion of the epidermis commences. In the human skin this is formed of three distinct layers. Undermost a layer of clear compressed cells, the stratum lucidum. Next above it a layer of swollen cells, the nuclei of which are indistinguishable. Finally, a surface layer of thin, horny scales, the stratum squamosum, which become detached and thrown off in the form of scurf or dandruff. In the skin of the horse, except where it is thickest, these layers are not clearly defined.
It is the Malpighian layer of the epidermis that is most active in cell division. As they are formed the new cells push upwards those already there, and the latter in their progress to the surface undergo a chemical change in which their protoplasm is converted into horny material. This change, as we have already indicated, takes place above the stratum granulosum.
In addition to its constant formation of cells to replace those cast off from the surface, the active proliferation of the elements of the Malpighian layer is responsible for the development of the various appendages of the skin, the hairs with their sebaceous glands, the sweat glands, horny growths and the hoof, and, in the human subject, the nail. These occur as thickenings and down-growths of the epithelium into the corium.
The epidermis is devoid of bloodvessels, but is provided with fine nerve fibrils which ramify between the cells of the rete mucosum.
THE CORIUM is composed of dense connective tissue, the superficial layer of which bears minute papillae. These project into the epidermis, which is moulded on them. For the most part the papillae contain looped capillary vessels, rendering the superficial layer of the corium extremely vascular. Why this must be a moment's reflection will show. The epidermis, as we have already said, is devoid of bloodvessels. It therefore depends entirely for its nourishment upon the indirect supply it receives from the vessels of the corium. The need for extreme vascularity of the corium is further explained when we call to mind the constant proliferation and casting off of the cells of the epidermis, the growth of the hairs, the production of the horn of the hoof, and the work performed by the numerous sweat and other glands.
Others of the papillae contain nerves, ending here in tactile corpuscles, or continuing, as we have mentioned before, to ramify as fine fibrils in the rete mucosum of the epidermis.
THE HAIRS are growths of the epidermis extending downwards into the deeper part of the corium. Each is developed in a small pit, the Hair Follicle, from the bottom of which it grows, the part lying within the follicle being known as the Root. It is important to note their structure, as it will be seen later that they bear an extremely close relation to the horn of the hoof.
Under a high power of the microscope, and in optical section, the central portion of a hair is tube-like. In some cases the cavity of the tube is occupied by a dark looking substance formed of angular cells, and known as the Medulla. The walls of the tube, or the main substance of the hair, is made up of a pigmented, horny, fibrous material. This fibrous structure is covered by a delicate layer of finely imbricated scales, and is termed the Hair Cuticle.
The root of the hair, that portion within the follicle, has exactly the same formation save at its extreme end. Here it becomes enlarged into a knob-like formation composed of soft, growing cells, which knob-like formation fits over a vascular papilla projecting up in the bottom of the follicle.
We have already stated that the hairs are down-growths of the epidermis. It follows, therefore, that the hair follicles, really depressions or cul-de-sacs of the skin itself, are lined by epithelial cells and connective tissue. So closely does the epidermal portion of the follicle invest the hair root that it is often dragged out with it, and is known as the Root Sheath. This is made up of an outer layer of columnar cells (the outer root sheath) corresponding to the Malpighian layer of the epidermis, and of an inner horny layer, next to the hair, corresponding to the more superficial layer of the epidermis, and known as the inner root sheath.
The hair grows from the bottom of the follicle by a multiplication of the cells covering the papilla upon which its root is moulded. When a hair is cast off a new one is produced from the cells covering the papilla, or, in case of the death or degeneration of the original papilla, the new hair is produced from a second papilla formed in place of the first at the bottom of the follicle.
THE SEBACEOUS GLANDS are small saccular glands with their ducts opening into the mouths of the hair follicles. They furnish a natural lubricant to the hairs and the skin.
THE SUDORIFEROUS OR SWEAT GLANDS are composed of coiled tubes which lie in the deeper portion of the skin, and send up a corkscrew-like duct to open on the surface of the epidermis. They are numerous over the whole of the body.
a, The nail; b, the rete mucosum; c, the longitudinal ridges of the corium.]
[Footnote A: Seeing that the section is a longitudinal one, it would appear from the way the ridges cut that they are running transversely beneath the nail. Their extreme delicacy, however, prevents a single one showing itself along the length of the section, and their constant accidental cutting makes them appear to run transversely (H.C.R.).]
THE HUMAN NAILS are thickenings of the lowermost layer of the horny portion of the epidermis, the stratum lucidum. They are developed over a modified portion of the corium known as the nail-bed. The horny substance of the nail is composed of clear horny cells, and rests immediately upon a Malpighian layer similar to that found in the epidermis generally. Instead of the papillae present elsewhere in the skin, the corium of the nail-bed is marked by longitudinal ridges, a similar, though less distinct, arrangement to that found in the laminae of the horse's foot.
Having thus paved the way, we are now in a better position to discuss our original question (Are the horny laminae secreted by the sensitive?), and better able to appreciate the work that has been done towards the elucidation of the problem.
A most valuable contribution to this study is an article published in 1896 by Professor Mettam.[A] Here the question is dealt with in a manner that must effectually silence all other views save such as are based upon similar methods of investigation—namely, histological examination of sections of equine hoofs in various stages of foetal development.
[Footnote A: The Veterinarian, vol. lxix., p.1.]
Professor Mettam commences by drawing attention to the error that has been made in this connection by studying the soft structures of the foot separated by ordinary putrefactive changes from the horny covering. "In this way," the writer points out, "a wholly erroneous idea has crept in as to the relation of the one to the other, and the two parts have been treated as two anatomical items, when, indeed, they are portions of one and the same thing. As an illustration, and one very much to the point at issue, the soft structures of the foot are to the horny covering what the corium of the skin and the rete Malpighii are to the superficial portions of the epidermis. Indeed, the point where solution of continuity occurs in macerating is along the line of the soft protoplasmic cells of the rete."
In the foregoing description of the skin we have seen that the corium is not a plane surface, but that it is studded by numerous papillary projections, and that these projections, with the depressions between them, are covered by the cells of the epidermis.
The corium of the horse's foot, however, although possessed of papillae in certain positions (as, for example, the papillae of the coronary cushion, and those of the sensitive frog and sole), has also most pronounced ridges (laminae) which run down the whole depth of the os pedis. Each lamina again carries ridges (laminellae) on its lateral aspects, giving a section of a lamina the appearance of being studded with papillae. We have already pointed out the ridge-like formation of the human nail-bed, and noted that, with the exception that the secondary ridges are not so pronounced, it is an exact prototype of the laminal formation of the corium of the horse's foot.
The distribution of the laminae over the foot we have discussed in the chapter devoted to the grosser anatomy. In a macerated foot the sensitive laminae of the corium interdigitate with the horny laminae of the hoof; that is to say, there is no union between the two, for the simple reason that it has been destroyed; they simply interlock like the unglued junction of a finely dovetailed piece of joinery. But no further, however, than the irregularities of the underneath surface of the epidermis of the skin can be said to interlock with the papillae of the corium does interlocking of the horny and sensitive laminae occur. It is only apparent. The horny laminae are simply beautifully regular epidermal ingrowths cutting up the corium into minute leaf-like projections.
In a macerated specimen, then, the exposed sensitive structures of the foot exhibit the corium as (1) the Coronary Cushion, fitting into the cutigeral groove; (2) the Sensitive Laminae, clothing the outer surface of the terminal phalanx, and extending to the bars; (3) the Plantar Cushion, or sensitive frog; and (4) the Sensitive Sole.
The main portion of the wall is developed from the numerous papillae covering the corium of the coronary cushion. We have in this way numberless down-growing tubes of horn. Professor Mettam describes their formation in a singularly happy fashion: "Let the human fingers represent the coronary papillae, the tips of the fingers the summits of the papillae, and the folds of skin passing from finger to finger in the metacarpo-phalangeal region the depressions between the papillae. Imagine that all have a continuous covering of a proliferating epithelium. Then we shall have a more or less continuous column of cells growing from the tip of the finger or papilla (a hollow tube of cells gradually moving from off the surface of the finger or papilla like a cast), and similar casts are passing from off all the fingers or papillae."
From this description it will be noticed that each down-growing tube of horn bears a striking resemblance to the growth of a hair, described on p. 47. In fact, the horn tube may be regarded as what it really is, a modified hair.
We next continue Professor Mettam's illustration, and note how the modified hairs or horn tubes become as it were matted together to form the hoof wall. The cells lining the depressions are also proliferating, and their progeny serve to cement together the hollow casts of the papillae, thus giving the inter-tubular substance. We have thus produced hollow tubes, united together by cells, all arising from the rete Malpighii of the coronary corium. Section of the lower part of the horn tubes shows them to contain a cellular debris.
Thus, in all, in the horn of the wall we find a tubular, an intertubular, and intratubular substance. In fact, hairs matted together by intertubular material, and only differing from ordinary hairs in their development in that they arise, not from papillae sunk in the corium, but from papillae projecting from its surface.
Although this disposes of the wall proper, there still confronts us the question of the development of the horny laminae. To accurately determine this point it is absolutely essential to examine, histologically, the feet from embryos.
In the foot of any young ungulate in the early stages of intra-uterine life horizontal sections will show a covering of epidermis of varying thickness.[A] This may be only two or three cells thick, or may consist of several layers. Lowermost we find the cells of the rete Malpighii. As some criterion of the activity with which these are acting, it may be noted that with the ordinary stains their nuclei take the dye intensely. The cells of this layer rest upon a basement membrane separating the epidermis from the corium. At this stage the corium has a perfectly plane surface.
[Footnote A: Equine foetus, seventy-seven days old.]
The next stage will demonstrate the first step in the formation of the sensitive laminae.[A] The plain surface of the corium has now become broken up, and what is noticed is that the broken-up appearance is due to the epithelial cells irrupting and advancing en echelon into its connective tissue. Each point of the ingrowing lines of the echelon has usually one cell further advanced into the corium than its neighbours, and may be termed the apical cell. The fine basement membrane separating epithelium from corium is still clearly evident. This epidermal irruption of the corium takes place at definite points right round the foot. It is extremely probable, however, that it commences first at the toe and spreads laterally.
[Footnote A: Sheep embryo, exact age unknown.]
As yet, these cellular ingrowths (which are destined to be the horny laminae, and cut up the corium into sensitive laminae) are free from irregularities or secondary laminae. Before these are to be observed other changes in connection with the ingrowths are to be noticed.
The first is merely that of elongation of the epithelial processes into the connective tissue, until the rete Malpighii gives one the impression that it has hanging to its underneath surface and into the corium a number of thorn-like processes. These extend all round the front of the foot, and even in great part behind. Accompanying this elongation of the processes is a condensation of the epithelial cells immediately above the rete Malpighii, with a partial or total loss of their nuclei. This is the first appearance of true horn, and its commencement is almost coincident with the first stages of ossification of the os pedis.
With the appearance of horn comes difficulty of sectioning. The last specimen that Professor Mettam was able to satisfactorily cut upon the microtome was from a foetus between three and four months old. In this the secondary laminar ridges were clearly indicated, and the active layer of the rete Malpighii could be traced without a break from one ingrowing epithelial process to the next, and around this, following all the irregularities of its outline, and covering the branches of the nascent laminae. The laminae mostly show this branching as if a number of different growing points had arisen, each to take on a function similar to the epithelial process as it at first appeared.
In the centre of the processes a few nuclei may be observed, but they are scarce, and stain only faintly; they have arisen from the cells of the rete Malpighii which have grown into the corium. In fact, the active cells are passing their daughters into the middle of the process, and these pass through similar stages as those derived from the ensheathing epidermis. In other words, the daughter cells of the constituents of the rete Malpighii which have grown into the corium pass through a degeneration precisely similar to that undergone by cells shed at desquamation, or those which eventually give rise by their agglutination to a hair.
This is the real origin of the horny laminae, and the thickness of these is increased merely by an increase in the area covered by the cells of the rete Malpighii—i.e., by the development of secondary laminar ridges. If a section from a foal at term be examined, the processes will be found far advanced into the corium, and, occupying the axis of each process, will be seen a horny plate, continuous with the horn of the wall. No line of demarcation can be observed between the horn so formed and the intertubular material of the wall. They merge into and blend with each other, with no indication of their different origins. The cells that have invaded the corium have thus not lost their horn-forming function. There has merely been an increase in the area for horn-producing cells. The horny processes are continuous with the hoof proper at the point where the epithelial ingrowth first commenced to invade the corium, and fuses here with the horn derived from the cells of the rete Malpighii which have not grown inwards, and which are found between the processes in the intact foot. From this it is clear that some considerable portion of the horn of the wall is derived from the cells of the rete Malpighii covering the corium of the foot. It becomes even more clear when we remember the prompt appearance of horn in cases where a portion, or the whole, of the wall has been removed by operation or by accident (see reported cases in Chapter VII.). |
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