Diseases of the Horse's Foot Part 2

[Ill.u.s.tration: FIG. 13.--MEDIAN SECTION OF FOOT. _A_, Os suffraginis; _B_, os coronae; _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 coronae; _I_, tendon of the flexor perforans; _J_, its pa.s.sing attachment to the os coronae; _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. Pa.s.sing down between the suspensory ligament in front, and the perforatus tendon behind, it glides over the sesamoid pulley and pa.s.ses through the ring formed by the perforatus.

Continuing its course, it pa.s.ses 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 p.r.i.c.ked 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 a.s.sists 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 pa.s.sing 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 pa.s.s 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 _circ.u.mflex artery of the coronet_.

[Ill.u.s.tration: FIG. 14.--THE ARTERIES OF THE FOOT. The digital; 2, the perpendicular--(_a_) its ascending branch, (_b_) its descending branch; 3, circ.u.mflex 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 circ.u.mflex artery of the toe); 5, the circ.u.mflex 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 coronae. Among the larger branches given off from its anterior portion are two descending, one on each side of the extensor pedis, to a.s.sist in the formation of the _Circ.u.mflex 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 pa.s.ses behind the circ.u.mflex artery of the pedal bone, and, during its pa.s.sage in the preplantar fissure, gives off ascending and descending branches, which ramify in the laminal tissue.

THE PLANTAR (UNGUAL[A]) ARTERY.--This, the larger of the two terminals of the digital, may be looked upon as a continuation of the main vessel.

Running along the plantar groove, it gains the plantar foramen. Here it enters the interior of the bone (the semilunar sinus) and anastomoses with the corresponding artery of the opposite side. The circle of vessels so formed is called the _Plantar Arch_ or the _Semilunar Anastomosis_.

[Footnote A: The epithet 'ungual' is added by Chauveau to distinguish these arteries from the properly so-called plantar arteries--the terminal divisions of the posterior tibial artery.]

From the semilunar anastomosis radiate two main groups of arterial branches, an ascending group and a descending one. The _ascending_ branches penetrate the substance of the os pedis, and emerge by the numerous foraminae on its laminal surface. The _descending_ branches, larger in size, also penetrate the substance of the pedal bone, and emerge in turn from the foraminae cribbling its outer surface--in this case the set of larger foraminae opening on its inferior edge. Having gained exit from the bone, their frequent anastomosis, right and left, with their fellows forms a large vessel following the contour of the inferior edge of the os pedis.

This const.i.tutes the _Circ.u.mflex Artery of the Toe_.

E. THE VEINS.

These commence at the foot with a series of plexuses, which may be described as forming (1) AN INTERNAL OR INTRA-OSSEOUS VENOUS SYSTEM, and (2) AN EXTERNAL OR EXTRA-OSSEOUS VENOUS SYSTEM.

1. THE INTRA-OSSEOUS VENOUS SYSTEM.--This is a venous system within the structure of, and occupying the semilunar sinus of the os pedis. It follows in every respect the arrangement of the arteries as before described in the same region. Efferent vessels emerge from the plantar foraminae, follow the plantar fissures, and ascend within the basilar processes of the os pedis.

Here they lie under shelter of the lateral cartilages, and a.s.sist 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 ca.n.a.l_ or ca.n.a.ls running along the bottom of the lateral lacunae of the plantar cushion to gain the deep layer of the coronary plexus. (2) _By the Circ.u.mflex or Peripheral Vein of the Toe_, a ca.n.a.l formed by ramifications from the solar and the podophyllous plexuses, and following the direction of the artery of the same name. The circ.u.mflex 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 circ.u.mflex 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 princ.i.p.al branches, which again unite to form two large vessels. These vessels, by their final fusion at the lower end of the first phalanx, const.i.tute 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, a.n.a.logous 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.

[Ill.u.s.tration: 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 circ.u.mflex 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 laminae.

_The Posterior Branch_.--This is the largest of the three, and may be regarded as the direct continuation of the plantar. At the fetlock it is placed immediately above the digital artery, but afterwards takes up a position directly behind that vessel. Together with the digital artery it descends to near the basilar process of the os pedis. Here it pa.s.ses with the plantar artery into the interior of the os pedis, and continues its main branch, with the preplantar artery, in the fissure of the same name, to finally furnish supply to the os pedis and the sensitive laminae. It is this nerve which is divided in the low operation of neurectomy.

Beyond the fact of this branch descending, in the region of the pastern, 1 inch behind the digital artery, a further point of interest presents itself to the surgeon, and one to which attention must be paid. This is the presence in close proximity to the nerve of the Ligament of the Pad (Percival), or the Ligament of the Ergot (McFadyean). This is a subcutaneous glistening cord originating in the ergot of the fetlock, pa.s.sing 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.