[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.

Fig. 37. The Dotted Line Represents The Change In Form Of The Coronary Edge Under Similar Circumstances

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.

Because of the shape of the cushion its change of form cannot be forwards (simultaneous pressure from above and below on to this wedge with its apex forwards must tend to give it a backward change of form). Because of the pastern being horizontal, and aiding in the downward pressure, its change of form cannot be upwards. And because of the ground it cannot be downwards. It follows, therefore, that the movement must be backwards and outwards, being especially directed outwards because of its shape and the median lacuna in its posterior half - this latter, the lacuna, accommodating as it does the frog-stay, preventing the tendency to backward movement becoming excessive, and directing the change of form to the sides. Where the greatest pressure is transmitted, then, is to the inner aspects of the flexible lateral cartilages. The coronary cushion being continuous with the plantar, the backward and outward movements of the latter will tend to pull upon and tighten the former, especially in front. This will account for the contraction noted by Lungwitz in the anterior half of the coronary edge of the hoof.

Remove the body-weight, and naturally the elastic nature of the lateral cartilages and the coronary and plantar cushions, with, in a less degree, that of the hoof, cause things to assume their normal position.

Repeat the weighting of the hoof, in this second case without frog-pressure, and we shall see at once that we have done away with one of the greatest factors in determining the outward and backward movements of the plantar cushion - namely, the pressure from below on its wedge-shaped mass. The movement of the plantar cushion will now be downwards as well as backwards; and, seeing that it is attached to the inner aspect of each lateral cartilage, we shall expect these latter, by the downward movement of the plantar cushion, to be drawn inwards. This Lungwitz has shown to occur.

The chief function of the lateral cartilages, therefore, is to receive the concussion engendered by locomotion, which concussion is directed backwards and outwards by the pad-like plantar cushion.

In addition to this, the lateral cartilages, together with the plantar and coronary cushions, play the part of a valve to the whole of the veins of the foot.

It is in this way: We have only to refer to the chapter on anatomy to see that the whole of the foot is covered with a tissue of extreme vascularity. Thus we find papillae - the over the coronary cushion; enlarged and modified papillae sensitive laminae - covering the anterior face of the os pedis; and numberless papillae again covering the sole. There can be no doubt that the quantity of fluid brought by the bloodvessels of these papillae to the foot acts largely as a means of hydraulic protection to the soft structures.[A] In like manner as that delicate organ, the brain, is best protected by being floated upon the cerebro-spinal fluid and bloodvessels (which fluids transmit waves of concussion or pressure through the organ without injury to the delicate cells forming it), so, in like manner, does the extreme vascularity of the foot protect the cells of its softer structures from the effects of pressure and concussion.

[Footnote A: The Veterinary Record, vol. iii., p. 518.]

That this law of hydraulics may operate in the horse's foot to the best

advantage, the veins must be provided with valves, and valves of no mean strength. These we know to be absent. It is here that the lateral cartilages and the elastic substances of the coronary and plantar cushions step in to supply the deficiency.

At the time when weight is placed upon the foot (with, of course, a tendency to drive the blood upwards in the limb), and, therefore, the time when a valvular apparatus is needed to retain the fluid in the foot, we find the wanting conditions supplied by the pressure outwards of the plantar cushion compressing the large plexuses of veins on each side of the lateral cartilages, to which plexuses, it will be remembered, the bulk of the venous blood from the foot was directed. A more perfect valvular apparatus, automatic and powerful, it would be difficult to imagine.