3 2 $ ZOOLOGY. 



The number of bones in these regions is very variable in 

 the phylum as a whole, but, in the higher forms particularly, 

 individuals of related species present remarkable uniformity. 



(The discussion of the condition of the skull and the origin of its 

 parts is entirely too technical for an elementary text. The student should 

 be referred to more advanced works.) 



342. The Appendicular Skeleton. Here are embraced 

 the skeletal parts of the appendages proper, together with the 

 bones binding them to the axial skeleton (girdles). Each 

 girdle may be said to consist typically of three bones, uniting 



FIG. 158. 



FIG. 158. Diagram of a trunk vertebra in a Mammal, c, centrum; ch., position 

 originally occupied by the notochord; h., head of the rib; /i.e., haemal cavity; n.a., 

 neural arch; n.c., neural canal; n.s., neural spine; r., rib; st., sternum; s.c., sternal 

 cartilage uniting ribs and sternum; t.p., transverse process of vertebra; tu., tubercle 

 of rib. 



Questions on the figure. Compare all the parts here with corre- 

 sponding ones of Figs. 157 : A, B, and note the differences. What is gained 

 by the articulation of ribs with a sternum? What is lost? In which 

 groups of Vertebrates is a sternum found? In which are fully developed 

 ribs found? 



to form a joint with the first bone of the limb. One of these is 

 dorsal and the others ventral (Fig. 159, B; il, is, p.). The 

 appendages are much alike both as to their girdles and the 

 limbs proper. The posterior is, in higher forms, more inti- 

 mately fused with the axial skeleton, thus securing greater 



