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 Coronæ (Anterior View)
3. The Os Coronæ (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
Haübner_)
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. Broué'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 Coronæ. (_Crawford_)
159. Fracture of the Os Coronæ. (_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 résumé 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 coronæ; 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 CORONÆ, 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.
[Illustration: FIG. 1.--THE BONES OF THE PHALANX. 1, The os suffraginis; 2,
the os coronæ; 3, the os pedis; 4, the navicular bone, hidden by the wing
of the os pedis, is in articulation in the position indicated by the barbed
line.]
[Illustration: FIG. 2.--SECOND PHALANX OR OS CORONÆ (ANTERIOR VIEW).
1, Anterior surface; 2, superior articulatory surface; 3, inferior
articulatory surface; 4, pits for ligamentous attachment.]
[Illustration: FIG. 3.--SECOND PHALANX OR OS CORONÆ (POSTERIOR VIEW). 1,
Posterior surface; 2, gliding surface for passage of flexor perforans; 3,
lower articulatory surface.]
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 foraminæ, 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 foraminæ 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 foraminæ. As the
name 'laminal' indicates, it is this surface which in the fresh state is
covered by the sensitive laminæ.
_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 foraminæ; 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 foraminæ.
_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.
[Illustration: FIG. 4.--THIRD PHALANX OR OS PEDIS (POSTERO-LATERAL VIEW).
1, Anterior or laminal surface; 2, preplantar foramen; 3, preplantar
groove; 4, basilar process of the wing; 5, retrossal process of the wing;
6, foramen caused by the ossifying together posteriorly of the basilar and
retrossal processes.]
[Illustration: FIG. 5.--THIRD PHALANX OR OS PEDIS (VIEWED FROM BELOW). 1,
Plantar surface; 2, plantar foramen and plantar groove; 3, semilunar crest;
4, tendinous surface; 5, retrossal processes of the wings.]
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 foraminæ, 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 coronæ, 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 coronæ. The articulating surface of the os coronæ 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 coronæ, 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.
[Illustration: FIG. 6.--THE NAVICULAR BONE (VIEWED FROM BELOW). 1, Inferior
surface (smooth for the passage of the flexor perforans); 2, anterior edge
of inferior surface; 3, posterior edge of inferior surface.]
[Illustration: FIG. 7.--THE NAVICULAR BONE (VIEWED FROM ABOVE, THE BONE
TILTED POSTERIORLY TO SHOW ITS ANTERIOR BORDER). 1, Superior articulatory
surface; 2, anterior border (grooved portion of); 3, anterior border
(articulatory portion of).]
[Illustration: FIG. 8.--LIGAMENTS OF THE FIRST AND SECOND INTERPHALANGEAL
ARTICULATIONS (VIEWED FROM THE SIDE). (AFTER DOLLAR AND WHEATLEY.) 1,
Outermost slip from the glenoidal fibro-cartilage; 2, lateral ligament of
the first interphalangeal articulation; 3, prolongations of the lateral
ligament of the first interphalangeal articulation attached to the end of
the navicular bone to form the postero-lateral ligament of the pedal joint;
4, end of the navicular bone; 5, antero-lateral ligament of the pedal
joint.]
_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.
[Illustration: FIG. 9.--LIGAMENTS OF THE FIRST AND SECOND INTERPHALANGEAL
ARTICULATIONS (VIEWED FROM BEHIND). (AFTER DOLLAR AND WHEATLEY.) 1,
Suspensory ligament; 2, innermost slip from complementary cartilage of
pastern joint; 3, middle slip from complementary cartilage of pastern
joint; 4, outermost slip from complementary cartilage of pastern joint; 5,
glenoid or complementary cartilage of pastern joint; 6, postero-lateral
ligaments of the pedal joint; 7, the navicular bone; 8, interosseous
ligaments of the pedal joint; 9, semilunar crest of os pedis; 10, plantar
surface of os pedis.]
_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 bursæ, 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.
[Illustration: FIG. 10.--THE FLEXOR TENDONS AND EXTENSOR PEDIS. (AFTER
HAÜBNER.) 1, Tendon of flexor perforans; 2, its supporting check-band from
the posterior ligament of the carpus; 3, tendon of the flexor perforatus;
4, ring and sheath of the flexor perforatus; 5, widening out of the flexor
perforatus to form the plantar aponeurosis; 6, suspensory ligament; 7,
reinforcing band from the suspensory ligament to the extensor pedis; 8, the
extensor pedis.]
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. 11.--THE FLEXOR PERFORANS AND FLEXOR PERFORATUS
TENDONS. The metacarpo-phalangeal sheath and the ring of the perforatus
laid open posteriorly, and the cut edges reflected to show the passage
of the perforans. 1, Reflected cut edges of the perforatus ring and the
metacarpo-phalangeal sheath; 2, the perforans tendon; 3, point of insertion
of the perforans tendon into the semilunar crest of the os pedis (this
widened and thickened extremity of the perforans is known as the plantar
aponeurosis).]
[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 coronæ.]
_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.
[Illustration: FIG. 13.--MEDIAN SECTION OF FOOT. _A_, Os suffraginis; _B_,
os coronæ; _C_, os pedis; _D_, navicular bone; _E_, tendon of the extensor
pedis; _F_, insertion of the extensor pedis into the pyramidal process of
the os pedis; _G_, the tendon of the flexor perforatus; _H_, insertion of
perforatus into the os coronæ; _I_, tendon of the flexor perforans; _J_,
its passing attachment to the os coronæ; _K_, its final insertion into the
semilunar crest of os pedis; _a_, section of coronary cushion; _b_, section
of plantar cushion; _c_, semilunar sinus of os pedis.]
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_.
[Illustration: FIG. 14.--THE ARTERIES OF THE FOOT. The digital; 2, the
perpendicular--(_a_) its ascending branch, (_b_) its descending branch;
3, circumflex artery of coronary cushion; 4, the preplantar (ungual)
artery--this is seen issuing from the preplantar foramen, and distributing
numerous ascending (_c_) and descending (_d_) branches (the latter concur
in forming the circumflex artery of the toe); 5, the circumflex artery
of the toe; 6, at the point marked (*) the terminal branch of the
digital--namely, the plantar ungual--is hidden behind the lateral
cartilage; 7, the lateral cartilage.]
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 coronæ. 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
foraminæ on its laminal surface. The _descending_ branches, larger in size,
also penetrate the substance of the pedal bone, and emerge in turn from
the foraminæ cribbling its outer surface--in this case the set of larger
foraminæ 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 foraminæ, 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 lacunæ 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.
[Illustration: FIG. 15.--THE VEINS AND NERVES OF THE FOOT. 1, The digital
vein; 2, its main tributaries, draining the podophyllous plexus, and
concurring to form the digital; 3. the digital artery (the main trunk only
of this is shown, in order to show its relationship with the vein and
nerve); 4, the plantar nerve, with its three branches--(_a_) the anterior
digital, (_b_) the middle digital, (_c_) the posterior digital; 5, the
podophyllous plexus; 6, superficial portion of the coronary plexus; 7, the
peripheral or circumflex vein of the toe.]
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 laminæ.
_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 laminæ. 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.
[Illustration: FIG. 16.--EXTERNAL FACE OF THE OUTER LATERAL CARTILAGE. 1,
External face of cartilage--(_a_) its upper border, (_b_) its posterior
border, (_c_) its anterior border, (_d_) its inferior border; 2, the os
pedis; 3, wing of os pedis.]
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 laminæ.
1. THE CORONARY CUSHION. In the foot stripped of the hoof the coronary
cushion is seen as a rounded structure overhanging the sensitive laminæ
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 laminæ.
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
papillæ, 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 laminæ.
[Illustration: FIG. 17.--THE KERATOGENOUS MEMBRANE (VIEWED FROM THE SIDE).
(THE HOOF REMOVED BY MACERATION.) 1. The sensitive laminæ, or podophyllous
tissue; 2, the coronary cushion; 3, the perioplic ring; 4, portion of
plantar cushion; 5, groove separating perioplic ring from coronary cushion;
6. the sensitive sole.]
The upper portions of the laminæ, 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 lacunæ of the pyramidal body, where they merge into the velvety
tissue of the sole and frog.
The papillæ of the coronary cushion secrete the horn tubules forming the
wall, and the papillæ of the perioplic ring secrete the varnish-like veneer
of thin horn covering the outside surface of the hoof.
[Illustration: FIG. 18.--THE KERATOGENOUS MEMBRANE (VIEWED FROM BELOW).
(THE HOOF REMOVED BY MACERATION.) 1, The sensitive sole; 2, the sensitive
frog[A]--(a) its median lacuna, (6) its lateral lacuna; 3. V-shaped
depression accommodating the toe-stay; 4, the sensitive laminæ which
interleave with the horny laminæ 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
lacunæ--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 papillæ. The cells clothing the
papillæ 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 papillæ, 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 LAMINÆ.--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 laminæ 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 _laminellæ_.
Examined on the foot, deprived of its horny covering, the sensitive laminæ
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 laminæ extend forward for a
short distance, where they interleave with the horny laminæ of the bars.
Much discussion has centred round the point as to whether or no the cells
of the sensitive laminæ 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.
[Illustration: FIG. 19.--THE WALL OF THE HOOF. 1, The toe; 2, inner toe; 3,
outside toe; 4, the quarter; 5, entigeral groove; 6, horny laminæ.]
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.
[Illustration: FIG. 20.--INTERNAL FEATURES OF THE WALL, FROG, AND SOLE
(MESIAN SECTION OF HOOF). 1, Horny laminæ covering internal face of wall;
2, superior border of wall; 3, junction of wall with horny sole; 4, the
cutigeral groove; 5, the horny sole; 6, the horny frog (that portion of
it known as the 'frog-stay'); 7, inverted V-shaped ridge on wall and sole
(known as the 'toe-stay'); 8, anterior face of wall; 9, inferior border of
wall.]
_The Internal Face_ of the wall, that adapted to the sensitive laminæ,
is closely covered over its entire surface with white parallel leaves
_(Keraphyllæ_, or horn leaves, to distinguish them from the _Podophyllæ_,
or sensitive leaves). These keraphyllæ dovetail intimately with the
sensitive laminæ, 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 papillæ 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 papillæ 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.
[Illustration: FIG. 21.--INFERIOR ASPECT OF HOOF. _a_ The inferior face of
horny sole; _b_, inferior border of the wall; _c_, body or cushion of the
frog; _d_, median lacuna of the frog; _e_, lateral lacuna of the frog; _f_,
the bar; _g_, the quarter; _h_, the point of the frog; _i_ the heel.]
_The External Border_ or Circumference is intimately dovetailed with the
horny laminæ 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 laminæ 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.
[Illustration: FIG. 22.--HOOF WITH THE SENSITIVE STRUCTURES REMOVED. 1,
Superior face of horny frog; 2, the frog-stay; 3, the lateral ridges of
the frog's superior surface; 4, the horny laminæ at the inflections of the
wall.]
_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 Lacunæ.
_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
laminæ 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.
[Illustration: FIG. 23.--VERTICAL SECTION OF EPIDERMIS (HUMAN). (AFTER
RANVIER) _A_, The horny layer of the epidermis; _B_, the rete mucosum;
_a_, the columnar pigment-containing cells of the rete; _b_, the polyhedral
cells; _c_, the stratum granulosum; _d_, the stratum lucidum; _e_, swollen
horny cells; _f_ the stratum squamosum.]
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 papillæ. These project into the epidermis, which is
moulded on them. For the most part the papillæ 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 papillæ 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.
[Illustration: FIG. 24.--SECTION OF SKIN WITH HAIR FOLLICLE AND HAIR. _a_,
The hair follicle; _b_, the hair root; _c_, the medulla; _d_, the hair
cuticle; _e_, the outer root sheath; _f_, the inner root sheath; _g_, the
papilla from which the hair is growing; _h_, a sebaceous gland; _i_, a
sudoriferous gland.]
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.
[Illustration: FIG. 25.--LONGITUDINAL SECTION THROUGH NAIL AND NAIL-BED OF
A HUMAN FOETAL FINGER.[A] _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 papillæ 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 laminæ 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 laminæ 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 papillæ in
certain positions (as, for example, the papillæ of the coronary cushion,
and those of the sensitive frog and sole), has also most pronounced ridges
(laminæ) which run down the whole depth of the os pedis. Each lamina again
carries ridges (laminellæ) on its lateral aspects, giving a section of
a lamina the appearance of being studded with papillæ. 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 laminæ over the foot we have discussed in the
chapter devoted to the grosser anatomy. In a macerated foot the sensitive
laminæ of the corium interdigitate with the horny laminæ 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 papillæ of the corium does interlocking of
the horny and sensitive laminæ occur. It is only apparent. The horny laminæ
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 Laminæ_, 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 papillæ
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
papillæ, the tips of the fingers the summits of the papillæ, and the folds
of skin passing from finger to finger in the metacarpo-phalangeal region
the depressions between the papillæ. 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 papillæ."
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 papillæ, 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 papillæ sunk in the corium, but from papillæ
projecting from its surface.
Although this disposes of the wall proper, there still confronts us the
question of the development of the horny laminæ. 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.]
[Illustration: FIG. 26.--SECTION OF FOOT OF EQUINE FOETUS, SEVENTY-SEVEN
DAYS OLD. The rete Malpighii rests on a plane corium; the rent in the
section is along the line of the cells of the rete (Mettam).]
[Illustration: FIG. 27.--SECTION FROM FOOT OF SHEEP EMBRYO. It shows a
pronounced epithelial ingrowth into the corium (Mettam).]
The next stage will demonstrate the first step in the formation of the
sensitive laminæ.[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 échelon_ into its connective
tissue. Each point of the ingrowing lines of the _échelon_ 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_
laminæ, and cut up the corium into _sensitive_ laminæ) are free from
irregularities or secondary laminæ. Before these are to be observed other
changes in connection with the ingrowths are to be noticed.
[Illustration: FIG. 28.--SECTION FROM CALF EMBRYO. The epithelial ingrowths
hang down from the epidermis into the corium like the teeth of a comb
(Mettam).]
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.
[Illustration: FIG. 29.--SECTION OF AN EPITHELIAL INGROWTH FROM AN EQUINE
FOETUS. It shows commencing secondary laminar ridges. In the centre are
epithelial cells which are undergoing change into horny elements to form
the horn core, or 'horny laminæ' (Mettam).]
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
laminæ. The laminæ 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 laminæ, 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.).
The activity of the cells of the rete Malpighii of the corium covering the
remainder of the foot will be quite as necessary as the activity of the
cells of the coronary papillæ which form the horn tubes themselves. 'For,'
in Professor Mettam's own words, 'I am inclined to believe that much of the
"white line" which is found uniting the wall of the hoof to the sole has
been derived from the horn formed from the rete of the foot corium. This
origin will explain the absence of pigment from this thin uniting "line,"
as it does from the horn lining the interior of the wall. The cells of the
rete are free of colouring matter.'
[Illustration: FIG. 30.--SECTION THROUGH HOOF AND SOFT TISSUES OF A FOAL AT
TERM. The horn of the wall is shown, and the horn-core ('horny laminæ') of
the epithelial ingrowth. The latter has advanced far into the corium, and
is now provided with abundant secondary laminar ridges (Mettam).]
From the matter here given us it is easy to understand how, in a macerated
foot, the appearance is given of interlocking of the sensitive and horny
laminæ. We see that the horny laminæ are ingrowths of the rete Malpighii,
ploughing into and excavating the corium into the shape of leaves--the
sensitive laminæ. Putrefactive changes simply break into two separate
portions what originally was one whole, by destroying the cells along its
weakest part. This part is the line of soft protoplasmic cells of the rete
Malpighii. Thus the more resistant parts (the horn on the one hand, and the
corium covering the foot on the other) are easily torn asunder.
As a result of the evidence we have quoted, we are able to answer our
original question in the affirmative. Seeing that the horny and the
sensitive laminæ are both portions of the same thing--namely, a modified
skin, in which the epidermis is represented by the horny laminæ, and the
corium by the sensitive--it is clear to see that the cells covering the
inspreading horny laminæ are dependent for their growth and reproduction
upon the cells with which they are in immediate contact--namely, those
of the sensitive laminæ--and that therefore the sensitive laminæ are
responsible for the growth of the horny.
B. CHEMICAL PROPERTIES AND HISTOLOGY OF HORN.
Horn is a solid, tenacious, fibrous material, and its density in the hoof
varies in different situations. It is softened by alkalies, such as caustic
potash or soda and ammonia, the parts first attacked being the commissures,
then the frog, and afterwards the sole and wall. Strong acids, such as
sulphuric acid and nitric acid, also dissolve it.
The chemical composition of the hoof shows it to be a modification of
albumin, its analysis yielding water, a large percentage of animal matter,
and materials soluble and insoluble in water. The proportions of these, as
existing in the various parts of the hoof, have been given by Professor
Clement as follows:
Wall. Sole. Frog.
Water 16.12 36.0 42.0
Fatty matter 0.95 0.25 0.50
Matters soluble in water 1.04 1.50 1.50
Insoluble salts 0.26 0.25 0.22
Animal matter 81.63 62.0 55.78
Horn appears to be identical with epidermis, hair, wool, feathers, and
whalebone, in yielding 'keratin,' a substance intermediate between albumin
and gelatine, and containing from 60 to 80 per cent. of sulphur.
That horn is combustible everyone who has watched the fitting of a hot shoe
knows. That it is a bad conductor of heat, the absence of bad after-effects
on the foot testifies.
[Illustration: FIG. 31.--PERPENDICULAR SECTION OF HORN OF WALL.]
In a previous page we have described the manner of growth of the horn
tubules, and noted the direction they took in the wall; also, we have
noticed the existence between them of an intertubular horn or cement.
Those who wish to give this subject further study will find an excellent
series of articles by Fleming in the _Veterinarian_ for 1871. We
shall content ourselves here with introducing one or two diagrams and
photo-micrographs, and dealing with the histology very briefly.
Under the microscope the longitudinal striation of the wall is found to be
due to the direction taken by the horn tubules.
Fig. 31 is a magnified perpendicular section of the wall. In it the
parallel dark striæ are the horn tubules in longitudinal section. The
lighter striæ represent the intertubular material.
Fig. 32 gives us the wall in horizontal section. To the left of this
picture we find the horn tubules cut across, and standing out as so many
concentrically ringed circles. In the centre of the figure are seen the
horny laminæ, with their laminellæ, and the sensitive laminæ. The right
portion of the figure pictures the corium.
[Illustration: FIG. 32.--HORIZONTAL SECTION OF HORN OF WALL.]
Fig. 33 is, again, a horizontal section, cut this time at the junction of
the wall with the sole. To the left are seen, again, the horn tubules of
the wall, and to the centre the horny laminæ. In this position, however,
the structures interdigitating with the horny laminæ are not sensitive, but
are themselves horny. As the diagram shows, they contain regularly arranged
horn tubules cut across obliquely. It is this horn which forms the 'white
line.' To the extreme right of the figure are seen the horn tubules of the
sole.
There remains now but to notice the arrangement of the horn tubules in the
frog. The peculiar, indiarubber-like toughness of this organ is well known.
Histological examination gives a reason for this.
[Illustration: FIG. 33.--HORIZONTAL SECTION OF HORN THROUGH THE JUNCTION OF
THE WALL WITH THE SOLE. _a_, Horn tubule of the wall; _b_, horn tubule of
the sole; _c, d_, horny laminæ.]
[Illustration: FIG. 34.--SECTION OF FROG THROUGH CORIUM AND HORN. The long
finger-like projections of corium into epidermis are sections of the long
papillæ from which the horn-tubes of the sole grow. In the stainable
portion of the epidermis are to be clearly seen light and dark streaks
pointing out the alternate strata-like arrangement of cells mentioned in
the text (Mettam).]
The horn tubules of the frog are sinuous in their course. This is accounted
for by the fact that in the horn of the frog there is a large amount of
intertubular material, this having the effect of frequently turning the
horn tubules from the straight. In addition to this, the intertubular
material has a peculiar arrangement of the cells composing it. These
are laid down in alternating striæ (1) of cells with their long axes
longitudinal, and (2) of cells with their long axes horizontal. This is
seen in Fig. 34, between the long papillæ of the corium, where the lines of
longitudinally arranged cells in horizontal section stand out darker than
the adjoining strata in which their arrangement is horizontal. The tortuous
direction of the horn tubules, and the almost interlocking nature of the
alternating strata of the intertubular material, together combine to give
the frog its characteristic toughness and resiliency.
C. EXPANSION AND CONTRACTION OF THE HOOF.
Among other questions productive of heated argument come those relating to
expansion of the horse's hoof. In the past many observers have strenuously
insisted on the fact that expansion and contraction regularly occur during
progression. Opposed to them have been others equally firm in the belief
that neither took place. Quite within recent times this question also
has been settled once and for all by the experiments of A. Lungwitz, of
Dresden. His conclusions were published in an article entitled 'Changes in
Form of the Hoof under the Action of the Body-weight.'[A]
[Footnote A: _Journal of Comparative Pathology and Therapeutics_, vol.
iv., p. 191. The whole of the matter in this article, from which we have
borrowed Figs. 35 and 36, is too long for reproduction here. It forms,
however, most instructive reading, and its careful perusal will well repay
everyone interested in this most important question (H.C.R.).]
In connection with this it is interesting to note how, all unconsciously,
two separate observers were simultaneously arriving by almost identical
means at an equally satisfactory answer to the question. Prior to the
publication of Lungwitz's article on the subject, Colonel F. Smith, A.V.D.,
had arrived at similar conclusions by working on the same methods.
[Illustration: Fig. 35. I. Electric Bell with Dry Element. a, Under
part, with box, for the dry element; 6, roller for winding up the
conducting-wires; c, dry element, with screw-clamp for attachment of the
conducting-wires; c', conducting-wire leading to the screw-clamp, with
contact-spring in c', Fig. 2, or to the wall in Fig. 3; d, upper part, with
bell; d', conducting-wire to the shoe d' in Figs. 2 and 3; e, strap for
slinging the apparatus around the body of the assistant or rider; f,
connecting-wire between bell and dry element.]
[Illustration: Fig. 35. II. Hoof Shod with Shoe provided with Toe-piece and
Calkins; Wall of the Hoof covered with Tinfoil. a, Heel angle, with b, the
contact-screws; c, screw-clamp, with contact-spring (isolated from
the shoe); c' conducting-wire from the same; d, screw-clamp, with
conducting-wire (d') screwed into the edge of the shoe; e, nails isolated
by cutting a small window in the tinfoil.]
[Illustration: Fig. 35. III. Hoof Shod with Plain Shoe; Horny Wall covered
with Tinfoil. a, Toe and heel angle, with b, the contact-screws; c,
conducting-wire passing from the tinfoil on the wall; d, conducting-wire
passing from the shoe; c', d', ends of the conducting-wires, which must be
imagined connected with the ends c', d', passing from the apparatus.]
It is unnecessary for our purpose here to minutely describe the exact
_modus operandi_ of these two experimenters. Briefly, the method of inquiry
adopted in each case was the 'push and contact principle' of the ordinary
electric bell, and the close attention which was paid to detail will be
sufficiently gathered from Figs. 35 and 36.
[Illustration: Fig. 36. I. LEFT FORE-FOOT SHOD AND MOUNTED TO RECOGNISE THE
SINKING OF THE SOLE. _a_, Iron plate covering the inner half of the horny
sole; _b_, openings in the same, with screw-holes for the reception of the
contact-screw _c_ (the part of the sole under the plate is covered with
tinfoil, which at _d_ passes out under the outer branch of the shoe, and
becomes connected with the tinfoil of the wall; in order to give the
freshly applied tinfoil a better hold, copying-tacks are at _e_ passed
through it into the horn, and one is similarly used to protect the tinfoil
at the place where the contact-screw touches the latter); _f_, holes
with screw thread for the fastening of the angle required to measure the
movement of the wall, and also for the fastening of the conducting-wire,
_g; h_, conducting-wire passing from the tinfoil; _i_, isolated nails.]
[Illustration: Fig. 36. II. BAR-SHOE WITH OPENINGS. _a_, Near the inner
margin and in the longitudinal bar; _b_, for the reception of the
contact-screw _c; d_, openings for fastening the angle and the
conducting-wires.]
After numerous experiments with the depicted contact-screws, moved to the
various positions indicated in the drawings, the following conclusions were
arrived at:
1. BEHAVIOUR OF THE CORONARY EDGE.--During uniform weighting of all four
hoofs the coronary edge shows a tendency to contraction in the anterior and
lateral regions of the hoof, and a tendency to expansion posteriorly. With
heavy weighting of the hoof, which is shown by a backward inclination of
the fetlock, contraction in the anterior and lateral regions is slight, but
the expansion behind, in the region of the heels, is distinct, commencing
gradually in front, becoming stronger, and diminishing again posteriorly.
The coronary edge of the heels becomes slightly bulged outwards. The bulbs
of the heels swell up and incline a little backwards and downwards.
When the fetlock is raised the expansion of the coronary edge of the heels
disappears from behind forwards, passing forwards like a fluid wave. In
the lateral and anterior regions of the coronary edge the contraction
disappears; and when the weight is thrown off the foot it passes into a
gentle expansion of the coronary edge of the toe. During the opposite
movement of the fetlock, that of sinking backwards, this change of form is
executed in the converse manner.
In short, the coronary edge resembles a closed elastic ring, which yields
to pressure, even the most gentle, of the body-weight, in such a way that a
bulging out of any one part is manifested by an inward movement of another
part.
In Fig. 37, _b_, the dotted line represents the changes of form in
comparatively well-formed and sound hoofs at the moment of strongest
over-extension[A] of the fetlock-joint.
[Footnote A: The term 'over-extension,' as employed by Lungwitz, is
intended to indicate that position assumed by the fetlock-joint when the
opposite foot is raised from the ground.]
2. BEHAVIOUR OF THE SOLAR EDGE.--Under the action of the body-weight this
is somewhat different from that of the coronary edge. Anteriorly, and at
the sides, as far as the wall forms an acute angle with the ground,
the tendency to expansion exists, but the change of form first becomes
measurable in the region where the lateral cartilages begin. Quite
posteriorly the expansion again diminishes.
Fig. 37, _a_, by the dotted line represents the expansion at the moment of
over-extension of the fetlock-joint. This expansion is itself rather less
than at the coronary edge, and it shows itself distinctly _only when the
weighted hoof is exposed to a counter-pressure on the sole and frog_, no
matter whether the counter-pressure is produced naturally or artificially.
Thus anything tending to the removal of the pressure from below, such as
a decayed condition of the frog or excessive paring in the forge, will
diminish the extent of expansion of the solar edge.
Contraction of the solar edge of the heels occurs at the moment of greatest
over-extension of the fetlock-joint--that is, in a foot with pressure
from below absent. On the face of it, this appears impossible. Lungwitz,
however, has perfectly demonstrated it; and, when dealing with the
functions of the lateral cartilages in a later paragraph, we shall show
reason for why it is but a simple and natural result of the foot dynamics.
3. BEHAVIOUR OF THE SOLE.--The horny sole becomes flattened under the
action of the body-weight. This is most distinct at the solar branches, and
gradually shades off anteriorly and towards the circumference. As might be
supposed, width of hoof and thickness of the solar horn exert an influence
on the extent of this movement. The sinking of the horny sole is most
marked in flat hoofs.
D. THE FUNCTIONS OF THE LATERAL CARTILAGES.[A]
[Footnote A: Extracted from a paper by J.A. Gilruth, M.R.C.V.S., in the
_Veterinary Record_, vol. v., p. 358.]
We have just referred to contraction of the heels as taking the place of a
normal expansion in those cases where ground frog-pressure was absent. We
shall readily understand this when we bear in mind the anatomy of the
parts concerned, especially that of the plantar cushion. This wedge-shaped
structure we have already described as occupying the irregular space
between the two lateral cartilages, the extremity of the perforans tendon,
and the horny frog.
Now, when weight or pressure is exerted from above on to this organ, and
the _frog is in contact with the ground below_, it is clear from the
position the cushion occupies that, whatever change of form pressure from
above will cause it to take, it must certainly be limited in various
directions.
[Illustration: FIG. 37. _a_, The dotted lines in this diagram represent the
expansion of the solar edge of the hoof at the moment of over-extension of
the fetlock-joint; _b_, the dotted line represents the change in form of
the coronary edge under similar circumstances.]
Because of the shape of the cushion its change of form cannot be forwards
(simultaneous pressure from above and below on to this wedge with its apex
forwards must tend to give it a backward change of form). Because of the
pastern being horizontal, and aiding in the downward pressure, its
change of form cannot be upwards. And because of the ground it cannot be
downwards. It follows, therefore, that the movement must be backwards and
outwards, being especially directed outwards because of its shape and the
median lacuna in its posterior half--this latter, the lacuna, accommodating
as it does the frog-stay, preventing the tendency to backward movement
becoming excessive, and directing the change of form to the sides. Where
the greatest pressure is transmitted, then, is to the inner aspects of the
flexible lateral cartilages. The coronary cushion being continuous with the
plantar, the backward and outward movements of the latter will tend to pull
upon and tighten the former, especially _in front_. This will account for
the contraction noted by Lungwitz in the _anterior half_ of the coronary
edge of the hoof.
Remove the body-weight, and naturally the elastic nature of the lateral
cartilages and the coronary and plantar cushions, with, in a less degree,
that of the hoof, cause things to assume their normal position.
Repeat the weighting of the hoof, in this second case _without
frog-pressure_, and we shall see at once that we have done away with one of
the greatest factors in determining the outward and backward movements of
the plantar cushion--namely, the pressure from below on its wedge-shaped
mass. The movement of the plantar cushion will now be _downwards_ as well
as backwards; and, seeing that it is attached to the inner aspect of each
lateral cartilage, we shall expect these latter, by the downward movement
of the plantar cushion, to be drawn _inwards_. This Lungwitz has shown to
occur.
The chief function of the lateral cartilages, therefore, is to _receive
the concussion engendered by locomotion_, which concussion is directed
backwards and outwards by the pad-like plantar cushion.
In addition to this, the lateral cartilages, together with the plantar and
coronary cushions, _play the part of a valve to the whole of the veins of
the foot_.
It is in this way: We have only to refer to the chapter on anatomy to see
that the whole of the foot is covered with a tissue of extreme vascularity.
Thus we find papillæ--the over the coronary cushion; enlarged and modified
papillæ sensitive laminæ--covering the anterior face of the os pedis; and
numberless papillæ again covering the sole. There can be no doubt that the
quantity of fluid brought by the bloodvessels of these papillæ to the foot
acts largely as a means of hydraulic protection to the soft structures.[A]
In like manner as that delicate organ, the brain, is best protected by
being floated upon the cerebro-spinal fluid and bloodvessels (which fluids
transmit waves of concussion or pressure _through_ the organ without injury
to the delicate cells forming it), so, in like manner, does the extreme
vascularity of the foot protect the cells of its softer structures from the
effects of pressure and concussion.
[Footnote A: The _Veterinary Record_, vol. iii., p. 518.]
That this law of hydraulics may operate in the horse's foot to the best
advantage, the veins must be provided with valves, and valves of no
mean strength. These we know to be absent. It is here that the lateral
cartilages and the elastic substances of the coronary and plantar cushions
step in to supply the deficiency.
At the time when weight is placed upon the foot (with, of course, a
tendency to drive the blood upwards in the limb), and, therefore, the time
when a valvular apparatus is needed to retain the fluid in the foot, we
find the wanting conditions supplied by the pressure outwards of the
plantar cushion compressing the large plexuses of veins on each side of the
lateral cartilages, to which plexuses, it will be remembered, the bulk
of the venous blood from the foot was directed. A more perfect valvular
apparatus, automatic and powerful, it would be difficult to imagine.
E. GROWTH OF THE HOOF.
We will conclude this chapter with a few brief remarks on the growth of
the hoof. That the rate of growth is slow is a well-known fact to every
veterinarian, and it will serve for all practical purposes when we state
that, roughly, the growth of the wall is about 1/4 inch per month. This
rate is regular all round the coronet, from which it follows that the time
taken for horn to grow from the coronary edge to the inferior margin
will vary according as the toe, the quarters, or the heels are under
consideration.
As might naturally be expected, the rate of growth will depend on various
influences. Any stimulus to the secreting structures of the coronet, such
as a blister, the application of the hot iron, or any other irritant,
results in an increased growth. Growth is favoured by moisture and by
the animal going unshod, as witness the effects of turning out to grass.
Exercise, a state of good health, stimulating diets--in fact, anything
tending to an increased circulation of healthy blood--all lead to increased
production of horn. With the effects of bodily disease and of ill-formed
legs and feet on the wear of the hoof, and the growth of horn, we shall be
concerned in a future chapter.
CHAPTER IV
METHOD OF EXAMINING THE FOOT