THE SKELETAL SYSTEM 49 



beyond a question as such, shows four complete toes and a sphnt ot a fifth. 

 The hind foot has three toes and a splint. These horses were about the size 

 of a fox terrier dog and known as "dawn horses." From this period the horse 

 gradually increased in height, the number of toes became reduced from four 

 to one on each limb, while the teeth increased in length and hardness with the 

 change in the character of the food. 



JOINTS 



A joint is the union of two or more bones or cartilages. Joints 

 are classified, according to structure, into three types : 



Type I, immovable, in which the opposed surfaces are directly 

 united by connective tissue, e. g., joints between the bones of the 

 skull. The periosteum also assists in this union. 





Fig. 7. — Diagram of section of true joint: /.Z., Fibrous layer; s.l., synovial 

 layer of joint capsule. The articular cartilages are white, bones dotted, and 

 joint cavity black in the figure. (Sisson, Anatomy of Domestic Animals.) 



Type II, slightly movable, in which the bones are united by 

 cartilage that is elastic enough to permit a slight movement, e. g., 

 the intervertebral joints. 



Type III, freely movable, in which a joint cavity exists be- 

 tween the opposed surfaces, e. g., hinge-joint, as that of the elbow; 

 and ball-and-socket joint, as that between the pelvis and the femur 

 at the hip. 



The freely movable joints are the only true joints (Fig. 7). 

 They have the following structures entering into their formation : 



1. Two or more bones the joint surfaces of which are usually 

 somewhat expanded. 

 4 



