Although the hoof is a firm, strong, protecting covering to the sensitive foot within it, very serious injury to the horse results from defects in its structure which are often overlooked. These will be appreciated more readily when it is known that within the hoof is a particularly delicate and complex arrangement. When a hoof is removed with care, a beautiful, sensitive structure is exposed, having a contour exactly matching the inner surface of the hoof (figs. 626, 627). The inner surface of the wall is covered with rows of thin, horny plates running from above downwards parallel to each other, all sloping forwards like the fibres of the wall. The corresponding portion of the sensitive foot presents hundreds of similar parallel projecting leaves of soft, velvety, fibrous tissue. These are called the sensitive laminae, and in the living-foot are dovetailed between the horny laminae of the wall so as to afford a firm, secure attachment between the two. The sensitive frog and sole are firmly attached to the corresponding horny parts, but instead of plates the connecting medium here is a mass of little papillae, so closely arranged as to give a velvety appearance and feel to the exposed surface. This sensitive layer, known to farriers as "the quick", is bountifully supplied with nerves and blood-vessels. Just where the hair meets the horn - the part called by horsemen the coronet - is a very important structure, seen when the hoof is detached. This is a prominent ring or band extending round the foot and covered with very large papillae. From it the wall grows, and injuries to it are followed by serious defects in the horn. Not only do such easily-recognized conditions as " sand-crack" and " false-quarter" follow injuries to the coronet, but all the defective qualities of horn, such as are found in dry, brittle hoofs, proceed from the coronet. So also do the rings and irregularities often noticed on the front of the hoof.
Fig. 626. - The Sensitive Foot: Side View.
A, Skin, a', Skin devoid of hairs. B, Peripolic band. c, Coronary cushion. D, Sensitive laminae.
Fig. 627. - The Sensitive Foot: Sole and Frog.
A, Median cleft of fleshy frog. B, La-minae of the bars. C, Velvety tissue of the frog. D, Velvety tissue of the sole.