Fig. 10.—Section at point.
The frog is fibrous, though not to such a marked degree as the other portions of the hoof. Its chief qualities are elasticity and toughness.
The Secreting Structures.
If we macerate a dead foot in water for a week or two, the hoof may be removed entire without injuring the tissues within. In this way the sensitive foot or "quick" is exposed to view, and presents an exact counterpart of the inside of the hoof. The sensitive foot consists of a layer of fibrous tissue stretched over the bones and other structures which form the centre of the foot. It is plentifully supplied with blood-vessels and nerves necessary to its double function as the source of horn growth and as the tactile organ of the foot. Horn is, of course, not sensitive, although the slightest touch on a horse's hoof is recognised by the animal, and this feeling is due to the impression made upon the sensitive foot. In the living horse any injury to the "quick" causes the greatest pain, and although this sensitiveness is a serious disadvantage in disease it is a most valuable provision in health, enabling the horse, even through a thick layer of horn, to recognise the quality of the surface upon which he may be standing or moving. It is this sense of touch—this tactile function—which demands that the sensitive foot should be so bountifully supplied with nerves.
Every farrier knows how profusely blood flows from any wound of the "quick"—evidence that the part is well supplied with blood-vessels. This full supply of blood is not merely for the ordinary waste and repair which takes place in every tissue; it is to meet a special demand—to supply the material for the production of horn. The sensitive foot is the secreting structure of the hoof, and the source of the constant growth and reproduction of horn. It corresponds with great exactness to the inside of the hoof, and as we have described the hoof in sections it may be convenient to follow that course with this structure, and to describe the sensitive frog, the sensitive sole, and the sensitive laminæ. We shall begin with the last.
The Sensitive Laminæ. Corresponding to the horny leaves on the inside of the wall, the sensitive foot presents an arrangement of minute parallel folds which are called the sensitive laminæ. ([Fig. 11]). Between these the horny laminæ rest, so that there is a kind of interleaved attachment which affords the very firmest connection between the wall and the sensitive foot. If the laminæ be laid bare in a living horse by removal of the wall, it is found that they have the power to secrete a kind of horn, not a hard fibrous horn like that of the wall, but a softer variety. This function is not very active in health or we should find that the lower edge of the wall was thicker than the upper; but it exists, and is very evident in some cases of disease.