STEUCTUBE OF A BONE. 



87 



in the movements of the joint have no periosteum, but are 

 covered by a thin layer of 

 gristle, known as the articu- 



Tmj 



Si 



I lar cartilage. Very early in 

 the development of the Body 

 the bone in fact was repre- 

 sented entirely by cartilage; 

 but afterwards nearly all this 

 was replaced by osseous tis- 

 sue, leaving only a thin carti- 

 laginous layer at the ends. 



The bone itself, Fig. 36, 

 consists of a central nearly 

 \ cylindrical portion or shaft, 

 extending between the dot- 

 ted lines x and z in the fig- 

 ure, and two enlarged artic- 

 ular extremities. 



On the upper articular ex- 

 tremity is the rounded sur- 

 face, Cp, which enters into 

 the shoulder -"joint, fitting 

 against the glenoid cavity of 

 the scapula; and on the low- 

 er are the similar surfaces, 

 Cpl and Tr, which articulate 

 with the radius and ulna re- 

 spectively. Besides carry- 

 ing the articular surfaces, 

 each extremity presents sev- 

 eral prominences. On the 

 upper are those marked Tmj 

 and Tm (the greater and 

 smaller trochanters), which 

 give attachment to muscles; 

 and similar eminences, the 

 external and internal con- 

 dyles, El and Em, are seen 

 on the lower end. Besides these, several bony ridges 



Cpl 



FIG. 36. The right humerus, seen 

 from the front. For description, see 

 text. 



