TYPES AND MARKET CLASSES OF LIVE STOCK 357 



they may undergo ossification, in which condition they are hard 

 and unyielding and are called sidebones. The plantar cushion 

 is a wedge-shaped mass of tough, elastic, fibro-fatty tissue fill- 

 ing all the space between the lateral cartilages, forming the 

 fleshy heels, and serving as a buffer to disperse shocks. It 

 extends forward underneath the navicular bone and flexor ten- 

 don, and protects these structures from injurious pressure from 

 below. 



Pododerm (or foot-skin). The pododerm or horn-produc- 

 ing membrane is merely a continuation of the derm, or true 

 skin. It covers the foot inside the hoof, just as a sock covers 

 the human foot inside the shoe. It differs from the ordinary 

 external or "hair" skin in having no sweat or oil glands, but, 

 like it, is richly supplied with blood vessels and sensitive nerves. 

 The functions of the pododerm are to produce the hoof and 

 unite it firmly to the foot. 



The hoof and how it grows. The horny shell, called the 

 hoof, which covers and protects the foot, is made up of three 

 parts, (1) the wall and bars, (2) the sole, and (3) the frog. 



Each part of the hoof is grown by some particular part or 

 parts of the pododerm. In general it may be said that the 

 horn of the hoof is made up of tubules or shafts of horn which 

 grow from papillae the same as does hair. These tubules are 

 cemented together by non-tubular matter corresponding to 

 dandruff exfoliated by the skin. In fact, so pronounced is the 

 similarity in growth of horn and hair that coarse hair, especially 

 on the legs and coronet, is associated with horn of coarse tex- 

 ture. Therefore the more coarse and brittle the hair about 

 the coronet, the more porous, brittle, and weak will be the forma- 

 tion of the horny hoof. 



The wall horn consists of three layers known as (1) the 

 outer or perioplic layer, (2) the middle or coronary layer, and 

 (3) the inner or laminous layer. The perioplic layer is very 

 thin. It is varnish -like in appearance and forms the surface 

 or crust of the wall. Its function is to preserve the moisture 

 of the foot and to absorb moisture. A horse working in sand 

 or dust usually has this outer layer worn away. The coronary 

 layer forms the real basis of the wall. At the heels it is deflected 

 forward to form the bars of the hoof. The angle between the 

 wall and bar is thickened and is called the buttress. The func- 

 tion of the bars is to react against contraction of the heels. 

 The inner or laminous layer of the wall horn is not tubular. It 



