32 VETERINARY LECTURES 



endosteum (unlike the periosteum) cannot be detached as a con- 

 tinuous membrane. In most birds, however, the medullary cavities 

 of the long bones contain air. 



48. Long" Bones are the weight-bearers, and are found in the 

 extremities or legs, and have a shaft and two ends. The compact 

 or hard structure exists on the outside, being thickest at the middle 

 and inner side of the shaft, or wherever most weight falls, there 

 covering the cancellated or porous and light portions. The latter 

 structure is most abundant at the ends of the bone, so as to give a 

 large yet light surface for joints, and it is there further covered by 

 articular cartilage. 



49. Flat Bones are found where important organs have to be 

 shielded. Thus, the bones of the head encase the brain ; the 

 shoulder-blade and ribs protect the heart and lungs, liver, etc. ; while 

 the hip or pelvic bones cover the organs of generation. Flat bones 

 are made up of two layers of compact tissue, with a layer of can- 

 cellated or porous tissue in the middle. 



50. Short and Irregular Bones are found in the backbone or 

 vertebral column, knee, hock, and lower portions of the limbs. 

 These bones are principally composed of cancellated tissue, covered 

 with a thin layer of the compact or hard structure, and are so 

 arranged as to bear weight, yet allow of movement, more particularly 

 of a gliding nature, and an ' open-and-shut ' motion. 



51. The Skeleton of the horse is said to be composed of about 

 253 bones (including the teeth), and is divided into head, trunk, 

 and extremities. These divisions will, perhaps, be better under- 

 stood by the following tables (Plate V.). 



52. The Head is subdivided into cranium and face. The 

 former consists of twenty separate bones, including four pairs and 

 four small bones in each ear ; while sixty-one bones are found in the 

 face — nine pairs, three single, and forty teeth in the horse, and 

 thirty-six in the mare. 



