STRUCTURE OF THE VERTEBRATES 



143 



but are normally worn down by w^ear against hard substances. 

 AYhen removed from their barren grazing lands the incisors of 

 llamas have been known to grow so long that the animals died 

 as a result. The canine tusks of the pig and some carnivores are 



Enamel- 

 Dentine 



Fig. 80. Structure of In- 

 cisor Teeth. Note the 

 structure of the open 

 pulp cavity which per- 

 mits the continued up- 

 ward growth of the 

 tooth. 



A. Closed Pulp Cavity 



B. Open Pulp Cavity 



similar in structure. The tusks of the elephant are the second 

 incisors of the upper jaw, lack enamel, and are the largest teetli 

 known. Certain extinct elephants had tusks eleven feet long, 

 weighing more than 250 pounds each. 



B. Visceral Skeleton 



The visceral, or branchial, skeleton is technically an entity, 

 although it forms an important part of the skull and can be 

 considered with the axial skeleton of the vertebrate. There is 

 no proved homologue of the branchial arches below the elasmo- 

 branch fishes. The numerous gill bars of Amphioxus afford no 

 clue as to the origin of vertebrate gill supports, while those of 

 the jawless cyclostomes are too highly modified to be of much 

 assistance. The latter group today has an intricate branchial 

 basket which has little relationship. with the gill arches of the 

 other vertebrates. 



In the most primitive sharks there are nine visceral arches 

 in the embryo, with rudiments of several more; but when the 

 number of gills was reduced to the typical five, the number of 

 arches was also reduced to seven. The anterior two of these be- 

 came modified into jaws and supporting structures of the throat 



