Fig. 11.—Foot with hoof removed showing at the upper part the Coronary band, and below the Sensitive Laminæ.
In laminitis the wall at the toe is often pushed forward out of position by a horny mass formed by the laminæ, and so we have the deformity of an excessive length of toe. In some cases of long continued sandcrack the irritation of the laminæ causes excessive secretion, and a horn tumour results. The sensitive laminæ, then, fulfil two functions; they offer a firm connecting medium for the wall, and they secrete horn. By the cruel experiment of removing the horny sole and frog of a living horse and then forcing him to stand on the maimed foot on a level surface, it has been shown that the laminæ are capable of alone supporting the weight of the animal. It has been argued from this that the laminæ always support the weight, and that the horse's foot may be described as being slung by the connecting laminæ. This is not true. The frog and sole help to support weight, and the hoof acts as one continuous whole, each part taking its direct and proportionate share of the weight placed upon the foot. The sensitive laminæ are not elastic, they are unyielding, and, therefore, allow no downward yielding which would impose excessive pressure on the sole.
The Coronary Band. (See [Fig. 11]). The sensitive laminæ do not cover the whole of the upright portions of the sensitive foot. There is between their upper extremity and the line which separates the skin from the sensitive foot, a convex band which runs round the upper border of the foot, and is turned downwards and inwards at the heels. This is called the coronary band, and corresponds to the groove which we noticed on the inner side of the upper border of the wall. On its surface are innumerable small projections or papillæ which, in the living animal, fit into the openings on the groove of the wall. From each of these papillæ grows a horn fibre, and from the surface between them is formed a softer horny matter—the two products forming together the substance of the wall. The coronary band is, then, an important structure, being the source from whence the wall is produced. Upon the healthy condition of this band depends the soundness of the wall, and any interference with its integrity must lead to defects or deformities in the wall.
The Sensitive Sole ([Fig. 12]) is that portion of the "quick" to which the sole is attached. Its surface is covered with papillæ, like those on the coronary band but much smaller, giving an appearance somewhat like the pile of velvet. From these the horn fibres of the sole are formed, and a firm means of connection is afforded for the floor of the hoof.
Fig. 12.—Under Surface of Foot showing Sensitive Frog and Sole.
The Sensitive Frog in structure resembles the sensitive sole, but its papillæ are very much smaller, and the surface therefore is smoother. The irregular prominent surface of the frog, with its cleft and the space at each side of it, is exactly reproduced on the sensitive frog, as might be expected, for the one is moulded on the other. There is one difference between the sensitive frog and the other portions of the sensitive foot which I may here mention. It is not attached to the bones of the foot except by its point, but is situated behind the bone, and has as a basis a mass of soft tissue which forms an important cushion or pad, to be referred to later.