THK IIORSK IN MOTION. 25 



them ; and if the proportion of the latter is increased, they are liable to 

 break. Variation from the normal proportions is the result of disease, 

 and is more common in the human family than among quadrupeds. 



The bones are covered with a compact, inelastic fibrous membrane, 

 the periosteum, which adheres so closely to their surfaces that consider- 

 able force is required to detach it. This membrane serves not only to 

 nourish the bones through its blood-vessels, or vascular system, but to 

 strengthen them and increase their elasticity. The Californian Indian 

 adopts the same method, for the same purpose, in the construction of 

 his bow. In studying the architecture of the skeleton, as a whole, it 

 will be found that no element of strength is wanting, or principle of 

 mechanics violated, in its structure. The bones are arched or bent 

 when such forms give greater strength. They are connected to each 

 other by a strong tissue, so flexible as to allow of the greatest free- 

 dom of motion, but inextensible, and, under all ordinary use, too strong 

 to be broken or detached from the bony levers whose motion it is 

 designed to limit. It is, however, sometimes torn, either completely 

 or partially, in dislocations or sprains ; and the slightest injury to this 

 tissue is a serious accident to an animal whose value depends on the 

 soundness of his locomotive organs. 



The extremities of bones which move upon each other, as at the 

 joints or articular surfaces, are covered with a peculiar formation 

 known as cartilage. It is insensible in a state of health, and very 

 elastic to pressure ; thickest where most exposed to concussion, and 

 covered with a membrane which secretes a glairy fluid adapted to 

 lubricate the opposing surfaces and reduce friction. These joints are 

 all closed to the admission of atmospheric air and all foreign sub- 

 stances, for their admission would soon cause serious injury. 



The joints are divided by anatomists into several classes, according 

 to their mechanical construction. Some are simple hinges, admit- 

 ting of motion in one direction only, as those of the lower parts of 

 the extremities. The heads of all the four columns of support are 

 provided with a kind of joint known to mechanics as the ball and 

 socket. This form admits of the greatest freedom of motion in every 

 direction ; but the motion is limited in extent by capsular ligaments 



