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"Diseases of the Horse's Foot"

Anatomy of the Horse's Hoof

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.

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.

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.

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.

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.

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.

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.

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).

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.

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.

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.

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).

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.

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.

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.

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.

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 broke 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.

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.

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.

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.

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.

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.

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.