DR. LEGEAR'S STOCK BOOK. 149 



CHAPTER XII. 

 BONES. 



The bones of a vertebrate animal form the skeleton (Plate I,.), or 

 frame-work upon which the body is built. There are about 216 

 separate bones; or including the teeth, 256 pieces in the skeleton 

 of a horse. These bones come together by means of joints, and 

 are held in place by ligaments, tendons, and muscles. Bones are 

 divided into long, flat, and irregular bones, for convenience of 

 description. Long bones are found principally in the legs, and 

 are composed of a very hard, compact tissue, in the centre of 

 which is a hollow, called the bone cavity, and filled with a sub- 

 stance called the marrow. In the marrow will be found nerves, 

 blood vessels, lymphatics, and fatty tissue. The bones in the 

 legs of a thoroughbred horse are smaller but stronger than those 

 of the heavy draft, coarse-boned animal. 



Bone is composed of earthy matter and animal matter. To 

 the earthy matter it owes its hardness, and to the animal matter 

 its toughness. The relative proportions of earthy and animal 

 matter vary according to age. In the young animal they are 

 about equal, but as age advances the earthy matter becomes 

 more abundant, and in old animals the bones become very brittle 

 and easily broken. In the trouble known as rickets, the earthy 

 salts are wanting, and, as the animal matter predominates, 

 the bones are soft and bend easily. Bones are surrounded by a 

 fine, fibrous enveloping membrane (the periosteum), which is 

 closely adherent to the external surface of the bone, and is, in 

 fact, the secreting membrane of the bony structure. Below will 

 be found a brief description of the various diseases and injuries 

 to which the bones of the body are liable. 



