114 



Basic Structure of Vertebrates 



The ventral ribs, in their relations to vertebrae and to the divi- 

 sions of the body-muscle, resemble the lateral parts of hemal arches. 

 Both are situated internally to the body-muscle. The ribs partially, 

 or sometimes almost completely, encircle the coelom and all of its en- 

 closed viscera, but never actually join ventrally. The hemal arch sur- 

 rounds the only visceral structures present in the tail — i.e., the caudal 

 blood-vessels — and is complete ventrally. 



Most fishes have well -developed ribs on all the trunk vertebrae and 

 even on the more anterior caudal vertebrae. In the land vertebrates, 

 the ribs tend to become more or less reduced or vestigial except in the 

 anterior trunk (thoracic) region, where they are an important part of 

 the mechanism of breathing. 



The sternum is a midventral skeletal structure situated in the 

 anterior region of the body-wall of the trunk. It develops as cartilage 

 and may be permanently cartilaginous, as in some amphibians and 

 many reptiles, or may become more or less completely ossified. It may 

 consist of one or several parts (Fig. 112). A dermal bone, the epi- 

 sternum or interclavicle, is joined to the anterior end of the sternum 

 of some reptiles (lizards and crocodilians) and of primitive mammals 



SUPRASCAI 



NTERCl^VICLE 



EPICORACOID 



PHOIO PROCESS 



Fig. 112. Types of vertebrate sterna. (A) Squalus. (B) Salamandra. (C) Nec- 

 turus. (D) Rana. (E) Felis. (F) Crocodilus. (G) Homo. The sternum is shown in 

 black. (Courtesy, Neal and Band: "Chordate Anatomy," Philadelphia, The 

 Blakiston Company.) 



