The lay mind does not readily grasp the fact that quite a considerable portion of the blood is circulating in the skin, and that one object of this is to ensure the rapid renewal of the skin, which being exposed to friction, would soon wear away to the sensitive parts unless constantly replaced. There are other functions the blood and skin performs, but these are not germane to our subject and may be neglected.

If the blood supply of any part of the body be cut off, the part dies. Whether it dies rapidly or slowly depends on the thoroughness with which the bloodstream has been cut off; if complete, the death of the part will only be a matter of a few hours, if incomplete it may take a few days. This statement holds good whether it be the question of a limb or a small piece of skin on the back.

Pressure will kill any of the tissues in the body; a leg may be completely amputated, skin, muscles, and bone, without the use of a knife or saw - a simple piece of elastic cord will do in the course of weeks what a knife can do in a minute or two. The pressure of the cord cuts off the blood supply, the part dies, and is gradually cut off.

The limb amputated by an elastic cord is an exaggerated example of the influence of pressure; for the purpose of the subject we are considering we dot have to think of any pressure greater than that produced by a man's weight, or any structure to die as a result of this pressure other than those found in the back.

The skin of the horse's back is subjected to pressure when the man mounts, and in consequence less bloods circulates through it than before. The greater the weight imposed the greater the pressure on the skin, and with every increase in pressure less blood is circulating through it.