CIRCULATORY APPARATUS. — The struc ture of the heart and the general arrangement of the arterial and VC7ZOUS systems offer no pecu liarities worthy of notice.
STRueTuRE OF TILE HORSE'S FOOT.—The mechanical structure of the foot of the horse demands to be considered at length, for in what ever point of view this part of their economy is regarded, either as a simple instrument of progression, or a curious piece of anatomy, it will be found equally deserving the study of the physiologist and of the veterinarian. Nu merous writers have accordingly devoted their attention to this subject, both on the conti nent and in our own country ; but theirige scriptions are, unfortunately, so mixed up with terms of farriery and stable jargon, that the anatomist finds considerable difficulty in deciphering their elaborate disquisitions. Among the most philosophical English treatises are those of Professor Coleman and Mr. Bracy Clark, to both of whom we shall be indebted for many of the following observations.
Horny The whole exterior conical covering of the horse's foot, called in tech nical language the " wall of the hoof," is formed of a dense horny substance, which in shape resembles a hollow cone obliquely truncated at its upper part, so that the hoof is deepest or highest in front of the foot, diminishing in this respect as it recedes back wards towards the " quarters ; " it then loses, to a considerable extent, its conical shape, and becomes nearly upright, especially on the inside or inner quarter, still growing narrower or lower to the posterior extrernity of the foot, where, at first sight, it appears to termi nate by mixing with the substance of the " frog," hereafter to be described, and 'with the integuments of the posterior region of the foot : instead of terminating in this manner, however, a more accurate examination shows it to be suddenly inflected inwards, pursuing its course towards the centre of the foot, where, diminishing gradually in depth, it is finally lost, becoming mixed up with the " sole," near the point of the frog, thus form ing a distinct and remarkable internal wall that supports the under parts of the foot, and at the same time protects, by its bold projection, the sole and the frog from an undue degree of pressure and contusion against the ground.
The parts thus formed by a continuation of the wall of the hoof beneath the foot, are called the "bars of the foot," and are fre quently described with, and taken for, part of the " sole." The direction of this sloping floor serves to throw all superincumbent pres sure outwards towards the sides of the foot, and at the same time leaves a triangular space, posteriorly, for the insertion of the frog (fig. 513.), which it likewise protects from inj ury.
The wall thus constructed appears to form the basis of the mechanism of the hoof, to which all the other parts are subordinate, and, if so understood, will much facilitate our views of the nature and economy of its structure. Its inner surface is every where lined, as it were, with numerous elastic lamellm that pro ject internally, and arranged in parallel lines, proceeding downwards perpendicularly to wards the front of' the foot (fig. 514, 3.) : these horny laminm are, at least, five hundred in number, and afford, from the aggregate surface that they present, a very extensive superficies for the attachment of an equal number of similar processes, derived from the vascular surface that covers the coffin bone, with which they interdigitate in such a way that the pres sure to which the foot is subjected, which if concentrated upon a small surface_would in evitably cause the destruction of living tissues, becomes so diffused as to produce no incon venient results.
The horny lamellae above alluded to, when removed from the hoof', have little or no elasticity when drawn in a longitudinal di rection ; but when drawn transversely, they possess this quality in a very remarkable de gree, more especially in resisting pressure ap plied in a direction outwards and downwards, to resist which, the arrangement of their fibres is, on close examination, found to be particu larly adapted.