Cartilages of the root. — On removing the hoof there are seen immediately beneath it two large elastic cartilages ranging to a great extent along both sides of the foot. Their hgure is almost too irregular for comparison ; but, when seen on a lateral view of the foot, their shape may be said to resemble that of a lozenge or of a pretty fully expanded fan, fixed by its centre, which is very much thicker and more solid than the other parts, in a deep horizontal cavity or channel in the coffin bone provided for its reception ; from this central point of insertion the anterior portion of it, passing forward, nearly meets the cartilage of the opposite side in front of the foot, the great extensor tendon of the foot only separating them, with which they are likewise connected, and make a common surface. On its inside this extremity of the cartilage takes a strong ad herence to the condyles of the coronary bone, and so closely surrounds the joint betwixt the latter and the coffin bone that the articulation appears to be without any capsular ligament at this part. The posterior portion of the cartilage, ranging more largely and becoming thinner as it expands itself backwards, growing at the same tune more elastic in its texture, is gradually and inseparably mixed up towards the hinder part of the foot with the skin and the ligamentary elastic tissues that form the "upper heels," and constitute the principal materials for elasticity in these parts. Spreading also in an upward direction to a considerable height above the hoof, it terminates by a rounded, thin, and irregular edge, which is inflected inwards over the soft interior of the foot, to which it forms a kind of roofing and defence.
Next, this widely distributed cartilage may be observed passing downwards, and surround ing on every side the rough and knotty ex tremities of the heels of the coffin bone, entering and filling up its sinuosities, and taking strong adherence to these processes ; it then extends itself horizontally inwards, passing over the horny sole and bar, and, meeting the side of the sensitive frog, inti mately unites with it, forming one inseparable mass, and together filling up the whole internal area described by the sides of the coffin bone. The upright or lateral portion of the cartilage forms, with this horizontal process inwards, a right angle, thus making together a hollow space or receptacle at the back of the coffin bone, that contains the spongy elastic stuffing of the heels, together with the tendons, trunks of bloodvessels, nerves &c., passing through this part to the sole of the foot. The upper surface of the horizontal process of cartilage is full of scabrous elevations and depressions that defy dissection, aniong which there exists a quantity of a gelatino-ligamentous material. Beneath, or to the under surface of this hori zontal layer of cartilage, the sensitive sole and bar are adherent ; and, in approaching the frog, or centre of the foot, it loses its carti laginous nature, and becomes coriaceous, or rather ligamento-coriaceous, in texture, agree ing in this with the internal frog.
- The horizontal portion or process of the cartilage, named by veterinary writers the "strat?form process," is of greater thickness and substance than the other parts; it is also of a coarser grain and more elastic nature ; both portions. together communicate the gene ral boundary and form to the lateral, the pos terior, and inferior parts of the foot ; and when the bars or frog are thrust upwards by pressure from without, they are then acting against this same horizontal flooring, formed by the cartilage and the frog, and met by the depression of the bones of the foot forced down from the weight of the animal; the whole can then dilate exteriorly along with the posterior and more elastic parts of the hoof.
The objects attained by the introduction of this admirable structure into the foot of the soliped are various, and have been well pointed out by Bracy Clark, in his excellent treatise, to which we must refer the reader for many practical applications connected with the veterinary art, that would be foreign to the objects of the present article. First, seeing that the resistance of a solid unyielding sup port would have been inadmissible, the pedal cartilages are employed as a substitute for bone, and made to occupy a very large share in the composition of the hinder part of the foot ; for it will be remarked, the coffin bone, except by its extremity, does not extend be yond the middle of the hoof (fig. 515.), the posterior shape of the foot being almost wholly communicated by the cartilage, which, passing nearly around the whole coronary circle, serves to support and convey the skin to its lodg ment in the coronary concavity of the hoof. Secondly, it serves to equalise the pressure every where over the internal surface of the hoof when under the pressure of the weight from within, during the descent of the bones of the foot, and, what is singular, the hoof itself is the most solid material of these hind parts of the foot.
A more iinportant office still remains to be explained, namely, that of supplying the coffin bone with a considerable share of its capability of motion in the interior of the hoof; for it is to be remarked that, as the coffin bone is obliged to describe in its de scent a small portion or segment of a circle, at its back part, round its centre of 'notion, or rather its more fixed part (for there is none of it wholly fixed), towards the front of the foot ; so this could not so well have been accomplished had the bone itself been fixed at its upper part to the processes in front of the hoof, these being too inconsiderable to afford, in that part of the bone, the extent of motion required ; but, by the intervention of an elastic cartilage between the bone and the substance of the hoof, the bone acquires greater liberty for action, and movement of its upper parts.
The cartilages of the foot, in old horses, not unfrequently become partially ossified, in which condition they are known to farriers by the name of ring-bones.
Soft Parts of the Foot. — On removing the hoof, and its horny appendages situated be neath the sole of the foot, the whole subjacent surface is found to consist of a thick, villous looking, and highly vascular membrane, moulded exactly to its inner surface, to which tbe name of sensitive foot is generally applied ; or, according to the structures beneath which it is situated, it is sometimes divided into sensitive hoof, sensitive frog, &c. This struc ture is, indeed, the matrix from which the entire corneous hoof derives its origin, and is essentially similar, in its texture and func tions, to the soft core upon which the hollow horns of many ruminants and the vascular secreting surfaces upon which the nails and claws of unguiculate quadrupeds are formed. Externally it presents, upon the anterior sur face of the foot, the broad vascular laminie which interdigitate with the horny plates, pro jecting from the interior of the hoof, as de scribed above, so as to amplify, to a very considerable amount, the extent of surface whereby the contact between the sensitive foot and the wall is effected.
This entire surface is richly supplied- with nerves and bloodvessels, the latter of which open into capacious plexuses, that surmount the coronary margin of the hoof (fig. 514.), and, when injected, present a very beautiful appearance.