THE BONES OF THE HOKSE. 345 



the free surface of all bones — that is, where they are not in con- 

 tact with other bones to form joints — is covered by an exceed- 

 ingly tough, thin membrane, somewhat resembling the inner 

 skin of an egg. This covering is called the periosteum. It 

 adds greatly to the strength of the bone, furnishes it protection 

 from injury by blows or kicks, and furnishes a bed for the 

 blood-vessels and nerves which supply the bone. It is a very 

 important substance. 



The bone itself is composed of a dense, hard outside, and a 

 softer, spongy inside, or pithlike structure, which is filled with 

 the marrow, or medullary substance. The larger ends of the 

 bones are not so dense and hard as the middle parts, particu- 

 larly of the long bones. 



Bones are composed of a mesh-work of a tough animal sub- 

 stance, which is filled in with the true bone matter, and which 

 is principally composed of lime. If the bone part is dissolved 

 away, there wdll still be left the shape of the bone, of a honey- 

 comb-feeling substance, which may easily be crushed by the 

 hand. On the contrary, the animal part of the bone may be 

 dissolved away and the true bone part left, very hard, and 

 brittle as a piece of limestone. 



The joints, or articulations, of the bones deserve some atten- 

 tion. It will be sufficiently accurate for the purposes of this 

 work to consider the articulations of the bones under three dif- 

 ferent classes, namely, the movable joints, the cartilage-joints, 

 and the fixed, or immovable, joints. 



The movable joints, or first class, embrace the ball and socket- 

 joints and the hinge-like joints. They admit of extensive mo- 

 tion in the bones thus united. These joints are held in place 

 by many strong elastic straps and bands, called ligaments. 

 They are the joints most likely to be sprained or thrown out 

 of place. For this reason they will be more carefully described 

 when speaking of the particular bones forming the diiferent 

 joints of this class. The joints of the limbs generally be- 

 long to this class. The ends of the bones in these joints are 



