2.  BEHAVIOUR OF THE SOLAR EDGE.—­Under the action of the body-weight this is somewhat different from that of the coronary edge.  Anteriorly, and at the sides, as far as the wall forms an acute angle with the ground, the tendency to expansion exists, but the change of form first becomes measurable in the region where the lateral cartilages begin.  Quite posteriorly the expansion again diminishes.

Fig. 37, a, by the dotted line represents the expansion at the moment of over-extension of the fetlock-joint.  This expansion is itself rather less than at the coronary edge, and it shows itself distinctly only when the weighted hoof is exposed to a counter-pressure on the sole and frog, no matter whether the counter-pressure is produced naturally or artificially.  Thus anything tending to the removal of the pressure from below, such as a decayed condition of the frog or excessive paring in the forge, will diminish the extent of expansion of the solar edge.

Contraction of the solar edge of the heels occurs at the moment of greatest over-extension of the fetlock-joint—­that is, in a foot with pressure from below absent.  On the face of it, this appears impossible.  Lungwitz, however, has perfectly demonstrated it; and, when dealing with the functions of the lateral cartilages in a later paragraph, we shall show reason for why it is but a simple and natural result of the foot dynamics.

3.  BEHAVIOUR OF THE SOLE.—­The horny sole becomes flattened under the action of the body-weight.  This is most distinct at the solar branches, and gradually shades off anteriorly and towards the circumference.  As might be supposed, width of hoof and thickness of the solar horn exert an influence on the extent of this movement.  The sinking of the horny sole is most marked in flat hoofs.

D. THE FUNCTIONS OF THE LATERAL CARTILAGES.[A]

[Footnote A:  Extracted from a paper by J.A.  Gilruth, M.R.C.V.S., in the Veterinary Record, vol. v., p. 358.]

We have just referred to contraction of the heels as taking the place of a normal expansion in those cases where ground frog-pressure was absent.  We shall readily understand this when we bear in mind the anatomy of the parts concerned, especially that of the plantar cushion.  This wedge-shaped structure we have already described as occupying the irregular space between the two lateral cartilages, the extremity of the perforans tendon, and the horny frog.

Now, when weight or pressure is exerted from above on to this organ, and the frog is in contact with the ground below, it is clear from the position the cushion occupies that, whatever change of form pressure from above will cause it to take, it must certainly be limited in various directions.

[Illustration:  FIG. 37. a, The dotted lines in this diagram represent the expansion of the solar edge of the hoof at the moment of over-extension of the fetlock-joint; b, the dotted line represents the change in form of the coronary edge under similar circumstances.]