CHAPTER III

EPIDERMIC PRODUCTS OF THE TERMINAL EXTREMITIES OF THE FORE AND HIND LIMBS

We will first recall to mind that among the quadrupeds some are found of which the fingers and toes have their third phalanges terminated by claws--these are the unguiculates; and that in others the terminal extremity of each limb is completely encased in a horny envelope, the hoof--these are the ungulates.

In the first group, the claws remind us to a certain extent of the arrangement of the nails in man; the inferior aspect of the paws is covered by an epidermic layer, thick and protective, which may be likened to the skin, correspondingly thick, which covers in the greater part of its extent the plantar surface of the foot in the human species.

In the second group, the surface by which the third phalanx rests on the ground is correspondingly protected, but this time by a layer of horn which belongs to the hoof.

After the preceding remarks, our study will be found to fall into a natural division, and it is in the order which we have just followed for the purpose of indicating its existence that we now proceed to study the nature and form of the different elements which complete or protect the digital extremities of the thoracic and abdominal limbs.

=Claws.=--These horny coverings of the third phalanges, which we have to consider only in the dog and cat, may be compared with the nails of man, with which, however, they present, as is well understood, characteristic differences.

The claws are compressed laterally, curved on themselves, and are terminated in front by a sharp point in the felide, but more blunted in the dog. Their superior border is convex and thick. We may say, therefore, that a claw is a sort of hollow tube, in the form of a cone flattened in the transverse direction, in which the third phalanx is set, and which is itself set in a groove formed by a kind of osseous hood which occupies the base of this third phalanx (see Fig. 37, p. 57).

This definition is exact, as regards the general appearance; but, when more closely scrutinized, it is not sufficient. The tube in question is not formed of a single piece; each of the claws is formed by a lamina laterally folded, but of which the borders are not exactly joined together inferiorly; they leave between them a small interval, and this is filled by a layer of more friable horny substance, to which has been given the name of plantar nail. This arrangement, which is clearly defined in the dog (Fig. 93), is comparable to that which we shall afterwards meet with in connection with the sole of the hoof of the horse (see Fig. 100, p. 257). In the dog and the cat, the weight of the limb resting on the inferior surface of the phalanges, it was necessary that the region of the plantar surface of the foot corresponding to these latter should be protected; this is the function of certain fibro-adipose pads, which are situated there, and which are designated by the name of _plantar tubercles_.

=Plantar Tubercles= (Fig. 94).--These tubercles, or dermic cushions, are divided, in each paw, into _tubercles of the digits_ (or of the toes), a _plantar tubercle_, and, on the fore-limbs, a _tubercle of the carpus_.

The tubercles of the fingers (or of the toes) are of the same number as the latter. That which belongs to the thumb is but little developed, but the others are more so. They are in relation with the plantar surfaces of the second and third phalanges, so that when the paw is in contact with the ground the articulation which, in each of the fingers or toes, joins these phalanges, rests on the corresponding pad.

The plantar tubercle is larger than the preceding. It is of a more or less rounded form; sometimes it is triangular, and then comparable in outline to the ace of hearts, the point of which is, in this case, turned towards the claws; its margin being sometimes strongly indented, it may also have a trilobate form. It is on it that rest the metacarpo-phalangeal or metatarso-phalangeal articulations, according to the limb studied. The tubercle of the carpus, situated at the level of the posterior surface of this latter, is less important than the preceding, the region which it occupies not reaching the ground during walking. But it is not to be neglected from the point of view of external form, because of the prominence which it produces.

In the ungulates the terminal extremity of the limb is, as we have above pointed out, enclosed in a horny envelope which is no other than the hoof.

We will first study the hoof of the horse--a hoof which is single for each of the limbs, inasmuch as in this animal each of these has but a single digit.

=Hoofs of the Solipeds.=--We will first study the hoof as regarded in a general way--that is, without taking into account the limb to which it belongs. We will afterwards point out the differences presented when the hoofs of the fore and hind limbs are compared.

In connection with the external forms of the horse, the study which we are now commencing is of great importance. But, before entering upon it, it appears to us necessary to rapidly examine what the hoof contains (Fig. 95).

In the interior of this horny box we find the third phalanx, a small sesamoid bone placed opposite to the posterior border of the latter, a portion of the inferior extremity of the second phalanx, and the tendons, which terminate at this region.

To the third phalanx are added two fibro-cartilaginous plates, flattened laterally, which prolong backwards the bone to which they are annexed. 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 assist 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.

The posterior and inferior borders of these cartilages meet at an acute angle. The angle so formed, or cartilaginous bulb, constitutes 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 constituent 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 horny 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 homogeneous 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.

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 _buttress_, is found the region of the _heels_. Finally, the portions of the wall which form its continuation in passing 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 buttress; 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.

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 constitutes 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 circumference of the sole (see further on).

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 pronounced. The constituent fibres from which this appearance results are directed from the superior border towards the inferior in parallel and regular lines.

The _sole_ is a horny 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 horny 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.

Indeed, these two horny structures appear to be homologous. 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 mass of horn, in form of a wedge, with its apex in front, which occupies the space 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 lacunæ_, or the _commissures of the frog_.

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.

With regard to the difference of form which we have just pointed out, we have sometimes heard the following comparison made: the contour of the hoofs of the fore-limbs, viewed from below, recalls that of an apple; that of the hoofs of the hind-limbs recalls the outline of a pear.

As a mnemonic this comparison is insufficient, for nothing connects either of the forms indicated with the region to which the hoofs belong.

We much prefer one made for us this very year by one of the students of our course at the School of Fine Arts, after the lecture in which we had just pointed out the differences in question. Giving the idea of a semicircle and an ogive, which we described above, he remarked to us that the idea would perhaps be more easily fixed in the memory if we associated with it the idea of the chronological order in which the Roman and ogival art succeeded. Indeed, as the Roman art preceded the ogival art, so the hoofs which have the semicircular form precede those which have the form of an ogive.

This interpretation appeared to us ingenious; this is why we wished to give it here a place which seems to us to be merited.

The wall of the hoof of a fore-limb, viewed on one of its lateral surfaces (see Fig. 98), is more oblique than that of one of the hind-hoofs looked at in the same way (Fig. 104). This difference, very marked especially at the region of the toe, is correlated with that of the direction of the pastern. In fact, in the anterior limbs this is a little more oblique than in the opposite ones.

We have still to describe, in connection with the horse, some epidermic tissues, which are known as _chestnuts_.

The chestnut is a small, horny plate which is found on the internal surface of each of the limbs, at a level differing on the anterior from that of the posterior ones.

On the anterior limbs the chestnut is situated on the internal surface of the forearm, towards the middle part, or the inferior third of this region. On the posterior limbs it is developed on the back of the superior extremity of the internal surface of the canon, towards the inferior part of the ham--that is, the tarsus.

Inasmuch as some authors consider the chestnuts as being vestiges of the thumb and the great-toe, we propose giving a mnemonic which will enable us to remember their situation, or, rather, their difference of level.

If we consider that the thumb, in the human species, is longer than the first toe, we may easily remember that the chestnut is placed higher in the anterior limbs than in the posterior ones. Indeed, if we suppose a digit taking its origin at these points, it will be longer in front (the thumb) than behind (the first toe).

=Hoofs of the Ox and the Pig.=--The ox has four hoofs on each foot--two which contain the third phalanges, and two others, rudimentary, situated at the posterior aspect of the limb, at the level of the inferior part of the canon; these latter bear the name of _spurs_. We will occupy ourselves especially with the former (Fig. 105).

Each of the hoofs presents three faces which, if we consider them in relation to the median axis of the limb to which they belong, are: external, internal, and inferior. The external surface resembles the wall of the hoof of the horse. The internal surface is slightly concave from before backwards, so that the external and internal hoofs of the same foot are not in contact with each other, except by the extremities of this surface, and that an interval separates them between these two points. The inferior surface, slightly depressed, ends behind in a swelling produced by the plantar cushion, which covers a thin lamina of horn.

At the anterior part of the hoof these three surfaces unite in forming a well-marked angle which, on account of the concavity of the internal surface, is slightly curved towards the axis of the foot.

The pig has also four hoofs--two for the great digits and two for the lateral digits. They recall those of the ox.