The inferior border of each of these fibro-cartilages is fixed by its anterior part to two osseous prominences situated at each of the angles which terminate the small phalanx behind; these prominences are: _the basilar process_ and _the retrorsal process_ (Fig. 96); by its posterior part, this border is continuous with a structure known as _the plantar cushion_ (see further on).
The posterior border is directed obliquely upwards and forwards. The superior border, which is convex or rectilinear, is thin, and is separated from the posterior border by an obtuse angle. Finally, the anterior border, which is directed obliquely downwards and backwards, is united to the ligamentous apparatus, which keeps the second and third phalanges in contact.
These fibro-cartilages, at their upper extremities, project beyond the hoof, and therefore a.s.sist in the formation of the lateral regions of the foot,[35] at the part which is called the _crown_. They project less above the hoof in the posterior limbs.
[35] Here, for the first time, apropos of the hoof, we use the word "foot." As in osteology and in myology we have, for the sake of clearness of comparison, designated under this name the region limited above by the tarsus, it is necessary to point out here that we employ the same word for a more restricted region. This we did in conformity with the usage of veterinarians, who so designate the region of the hoof. It is necessary to explain this double employment of the word, and, further, to show the particular meaning ascribed to it.
[Ill.u.s.tration: FIG. 96.--THIRD PHALANX OF THE HORSE: LEFT ANTERIOR LIMB, EXTERNAL SURFACE.
1, Pyramidal eminence; 2, surface, for articulation with the inferior extremity of the second phalanx; 3, basilar process; 4, retrorsal process.]
The posterior and inferior borders of these cartilages meet at an acute angle. The angle so formed, or cartilaginous bulb, const.i.tutes the base of the region, which is commonly called the _heel_--a part of the foot which, as its name implies, is situated posteriorly, but which we must not confound, as we might be led to do, with the region occupied by the calcaneum. We know from our previous studies of comparative osteology that this latter is situated much higher up.
The _plantar cushion_ is a sort of fibrous wedge which occupies the interval bounded by the fibro-cartilaginous plates which we have just been studying. Its inferior surface, the form of which we shall find to be reproduced by a portion of the corresponding surface of the hoof, is prolonged anteriorly into a point, while behind it is divided into two branches, which, diverging from one another, join the posterior angles of the fibro-cartilages. These two branches are separated by a median excavation.
The different const.i.tuent elements which we have just been discussing give elasticity to the foot.
To finish the examination of the parts contained in the hoof, we will add that among them is also found what is called the fleshy _envelope_, or _flesh_ of the foot.
We divide the latter into three regions: the podophyllous tissue, striated or laminated flesh which is spread out over the anterior surface of the third phalanx; the pad, or the hardened skin which corresponds to the upper border of the hoof, and forms a prominence above the podophyllous tissue; and the villous flesh, or velvety tissue which covers the plantar surface of the third phalanx and the plantar cushion. These three tissues form as a whole the keratogenic membrane--that is to say, that which produces h.o.r.n.y tissue, and consequently regenerates the hoof.
It is this latter that we now proceed to study.
When we examine its anterior surface or the opposite one, the hoof of the horse has the shape of a truncated cone with the base below and the summit cut off obliquely downwards and backwards (Fig. 97).
Viewed on one of its lateral aspects, it may be compared to a truncated cylinder placed on the surface of the section (Fig. 98). We particularly call attention to this latter comparison, for it singularly aids us in making a representation of the foot of the horse when viewed laterally.
Notwithstanding that the hoof forms apparently a h.o.m.ogeneous whole, it consists of three parts, which may be separated from one another by maceration. The indication of such disunion, artificially produced, may seem useless. It is not so, however, for this division of the hoof will permit us to carry out the study of the latter in a clearer, and consequently a more satisfactory, way. The three parts in question are the _wall_, or _crust_, the _sole_, and the _frog_.
The _wall_ is that portion of the hoof which we see when the foot rests on the ground. It is a plate of horn which, applied to the anterior and lateral surfaces of the foot, diminishes in height as it approaches the posterior part of the region. Posteriorly and at each side the wall is folded on itself, and is then directed forwards to terminate in a point, after having enclosed the frog, which we will soon study.
[Ill.u.s.tration: FIG. 97.--LEFT ANTERIOR FOOT OF THE HORSE: ANTERIOR ASPECT.
1, Outer side; 2, inner side.]
Although the wall forms a continuous whole, it has been divided into regions to which special names are given. The anterior part, from the superior border to the inferior, is called the _pince_ or _toe_ for a width of 4 to 5 centimetres. External to the toe, and on each side of it, for a distance of 3 or 4 centimetres, is the _nipple_. Behind the _nipples_ are the _quarters_. Still further back, where the wall folds on itself, forming the _b.u.t.tress_, is found the region of the _heels_.
Finally, the portions of the wall which form its continuation in pa.s.sing forward are called the _bars_.[36] These are only visible on the inferior surface of the hoof (see Fig. 100).
[36] It is to the angle of inflexion or heel that some authors give the name of b.u.t.tress; it is the bars which other authors designate in this fashion.
The wall, convex transversely, is, in its anterior part (viz., the _toe_) inclined strongly downwards and forwards. This obliquity tends to become gradually effaced on the lateral parts to such a degree that at the quarters it becomes almost perpendicular to the surface of the ground.
The internal quarter is less rounded than the external; in addition to this (Fig. 97), it approaches more nearly to the vertical direction.
[Ill.u.s.tration: FIG. 98.--LEFT ANTERIOR FOOT OF THE HORSE: EXTERNAL ASPECT.
1, Fetlock; 2, spur or beard; 3, pastern; 4, outline determined by the external fibro-cartilage; 5, acute angle; 6, nipple; 7, quarter; 8, heel.]
In our opinion, this latter difference clearly recalls certain characters of the general form of the human foot. In fact, the latter has its dorsal surface inclined downwards and outwards, whereas its internal border may be said rather to be vertical. A transverse section of the foot (Fig. 99) justifies this comparison, which to us appears interesting, not only as regards the resemblance which exists between these organs of support, but, further, because it const.i.tutes a mnemonic which enables us, on condition that we remember the form of the human foot, to recall the above-described character of that of the horse.
The greater convexity of the outer portion of the hoof is found equally on the human foot; the external border of this foot is more convex than the opposite one.
The inferior border of the wall (Fig. 100) is, in the case of unshod horses, always in wear when in contact with the ground. It is intimately united to the circ.u.mference of the sole (see further on).
[Ill.u.s.tration: FIG. 99.--VERTICAL AND TRANSVERSE SECTION OF A LEFT HUMAN FOOT: OUTLINE OF THE DIVIDED SURFACE OF THE POSTERIOR SEGMENT OF THIS SECTION (DIAGRAMMATIC FIGURE).
AA", Vertical axis pa.s.sing through the middle of the leg and the second toe; 1, outer side; 2, inner side.]
The superior border is hollowed on its internal surface by a groove, the cutigerous cavity or basil, which lodges the cushion (see Fig. 95). We have described this latter above, in connection with the flesh of the foot.
The substance of the wall presents a fibrous appearance which is pretty strongly p.r.o.nounced. The const.i.tuent fibres from which this appearance results are directed from the superior border towards the inferior in parallel and regular lines.
The _sole_ is a h.o.r.n.y plate which occupies the inferior surface of the hoof (Fig. 100). It is situated between the inferior border of the wall and the bars; and, on account of the oblique direction of these latter, it presents a strongly-marked groove of a [V]-form, with the opening directed backwards. In this depression is lodged the frog.
The inferior surface is concave, and thus forms a sort of vault, more or less deep, according to the individual. The sole has a scaly, laminated aspect.
We have seen (Fig. 93, and p. 249) that on the inferior surface of the claws of carnivora is found a small interval which is filled by a plate of a more friable h.o.r.n.y substance, to which has been given the name of the plantar nail. It seems to us that there is an interesting relationship between the said plantar nail and the sole which we have just been studying.
[Ill.u.s.tration: FIG. 100.--INFERIOR SURFACE OF A FORE-HOOF OF THE HORSE: LEFT SIDE.
1, Internal border of the wall (toe); 2, wall; 3, quarter; 4, heel; 5, bar; 6, sole; 7, frog; 8, median cavity; 9, prominence of the frog; 10, lateral cavity.]
Indeed, these two h.o.r.n.y structures appear to be h.o.m.ologous. Is not the lamina of the claw comparable to the wall of the hoof? And does not the interval which occurs at the inferior part of this latter, and is filled by the sole, recall that which, in extremely reduced form, is filled by the plantar portion of the claws?
The _frog_ (Fig. 100) is a ma.s.s of horn, in form of a wedge, with its apex in front, which occupies the s.p.a.ce limited laterally by the recurved portions of the wall (the bars) and the posterior border of the sole.
It covers the plantar cushion previously described (p. 252) and reproduces its form.
Its inferior surface is hollowed out in the middle by an excavation, which is known as the _median lacuna_. This cavity separates the branches of the frog, which terminate posteriorly by two swellings which are known as _the prominences of the frog_, forming two rounded elevations situated above the claws. These same branches unite in front of the median lacuna to form the body of the frog. This latter, in its anterior part, gradually narrows, and terminates in a point which occupies the bottom of the hollow limited laterally by the bars of the wall and the posterior border of the sole.
Between the lateral surfaces of the frog and the bars are found two angular cavities--_the lateral lacunae_, or the _commissures of the frog_.
[Ill.u.s.tration: FIG. 101.--THIRD PHALANX OF THE HORSE: LEFT ANTERIOR LIMB, INFERIOR VIEW.
1, External border; 2, internal border; 3, semilunar crest; 4, 4, re-entrant processes.]
[Ill.u.s.tration: FIG. 102.--THIRD PHALANX OF THE HORSE: LEFT POSTERIOR LIMB, INFERIOR VIEW.
1, External border; 2, internal border; 3, semilunar crest; 4, 4, re-entrant processes.]
As an indispensable complement to the study which we have just made, it is necessary to add that the hoofs of the fore-limbs and those of the hind ones present differences of form which cannot be ignored--differences which we are already able to conjecture by looking at the respective third phalanges which terminate those limbs, and especially at their inferior surfaces (Figs. 101, 102).
The hoofs of the fore-limbs (see Fig. 100), viewed on their plantar surface, are more rounded than those of the hind-limbs (Fig. 103)--so that their external contour may be compared to a semicircle--whilst the hind-hoofs, which are narrow and of more oval shape, rather recall by their form the aspect of an ogive.
This seems to result from the fact that the fore-limbs support the more considerable part of the weight of the animal. The best proof which can be given of this overweighting is the eagerness with which very often, when a horse is stopped near the edge of a footpath, for example, he places his fore-feet on the latter. In thus raising his fore-quarters, he throws part of his weight backwards, and in this way relieves his fore-limbs.
[Ill.u.s.tration: FIG. 103.--INFERIOR SURFACE OF A HIND-HOOF OF A HORSE: LEFT SIDE.
1, External border; 2, internal border.]