102 



Basic Structure of Vertebrates 



cellular tissue which owes its supporting capacity to the fact that it is 

 enclosed by a tough and elastic fibrous sheath. Cartilage is a solid 

 material somewhat resembling moderately hard rubber in consistency 

 and texture and in being usually somewhat elastic. It consists essen- 

 tially of a complex protein substance secreted by cells which remain 

 permanently embedded in it. Bone consists mainly of calcareous sub- 

 stance, which makes it harder and more rigid than cartilage. Its his- 

 tologic structure is much more complex than that of cartilage (see 

 pp. 323-327). 







- - - _ ! -J2&, STAGES 



DEGENERATION 



OSTEOCLASTS 



OSTEOBLAST 



Fig. 98. Formation of endochondral bone at the end of a long bone. Destruc- 

 tion of cartilage is followed by the deposition of lime in the form of thin lamellas. 

 Osteoblasts then lay down bone upon these lamellas. In this way cancellous bone 

 replaces cartilage. (After Dahlgren and Kepner. Courtesy, Neal and Rand: 

 "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



