i=;o 



ELEMENTS OF BIOLOGY 



The Human Skeleton. The human endoskeleton (Fig. 98) 

 is composed of more than two hundred bones, some of which are of 

 exoskeletal origin, being dermal bones, but most are cartilage bones. 

 In fact, the human skeleton constitutes a most impressive record 



COff/i CO ID 

 REGION 



CLAVICLE 

 FORELIMB JOINT 

 SCAPULA REGION 

 SCAPULA — 



CARTI LEGES OF FIN 



CORACOID 



DOGFISH 



FROG 



FORELIMB 

 JOINT 



CLAVICLE 



ARM JOINT 



SCAPULA 



CORA CO ID 



TURTLE MAN 



Fig. 99. — Shoulder girdles of some vertebrates. The scapula and coracoid are 

 derived from the primitive cartilage girdle, shown in the dogfish. The clavicle is a 

 membrane or dermal bone. In the human the coracoid exists as a small protuberance 

 on the scapula. 



of changes brought about by the incorporation of parts of the 

 primitive exoskeleton into the endoskeleton; its various bones can 

 be related to bones, cartilages, and dermal scales of lower forms. 

 For example, the shoulder blade or scapula is a cartilage bone 

 of primitive origin among vertebrates, for its relation to a car- 

 tilage bar in the lower fishes may be traced with some accuracy 



