48 



THE HUMAN BODY. 



Cp 



FIG. 21.-The 

 from the front, 

 text. 



r description Tee 



which dies if it be stripped 

 off.* The periosteum covers 

 the humerus except on its 

 ends (Cp, Tr, Cpl) at the 

 shoulder and elbow- joints ; 

 there the bone is covered by 

 a thin layer of gristle or car- 

 tilage. Very early in life the 

 whole humerus consists of 

 cartilage ; this is afterwards 

 absorbed and replaced by 

 bone, leaving only a thin 

 layer of articular cartilage 

 on each end. 



The bone itself consists 

 of a central nearly cylindrical 

 portion or shaft (extending 

 between the dotted lines X 

 and Z) and two articular ex- 

 tremities. These extremities 

 are enlarged to give a wider 



What are the functions of the 

 periosteum? Where is the perios- 

 teum absent? Of what does the 

 humerus consist in very early in 

 life? What happens to most of its 

 cartilage afterwards ? Where is 

 some cartilage left? 



What are the main divisions of 

 the humerus? What is the general 

 form of its shaft ? Why are its ar- 

 ticular extremities large? 



* Cases have been recorded in which a 

 considerable portion of a bone or even the 

 whole bone has been removed during life, 

 and the periosteum O ef t but slightly in- 

 jured) has formed a new bone in place of 

 the old. 



