CH. VIIl] FROGS, TOADS, AND NEWTS 267 



3. Inspiration (Injection). Nostrils closed or open ; 

 glottis open; air forced into lungs by petroliyoids and 

 other muscles, while some escapes through nostrils. 



The Nervous System and Aocial Skeleton. The nervous 

 system of the frog is far more complicated, and the 

 functions of its component parts more fully understood 

 than in the case of any of the preceding animals described 

 in this volume. It consists of a well-developed brain 

 and spinal cord from both of which nerves proceed to the 

 sense organs, muscles, and other viscera. The brain is 

 enclosed within the cranium, which is an almost complete 

 tube of cartilage. In parts it is ossified, elsewhere plates 

 of bone, developed in membrane, are laid down outside 

 the cartilage. The brain is thus adequately protected 

 from external shock, and a firm axis is afforded upon which 

 the bones of the jaws are fixed, and on which those which 

 are moveable find fulcra more or less directly in their 

 movements as levers : it also serves as a fixed base upon 

 which the muscles of the eyes and jaws and other anterior 

 parts of the body can pull. In the same way the spinal 

 cord, which is continuous with the hinder part of the 

 brain, is protected by the vertebra of the "backbone." 

 During early life the spinal cord is supported by the 

 notochord ; subsequently this flexible rod-like structure is 

 supplanted by the rigid bony centra of the nine vertebrse 

 and the unsegmented urostyle. Flexibility of the trunk 

 is however retained by the free articulations of the 

 vertebras one with another. From the centra there 

 spring corresponding arches of bone the' neural arches 

 which enclose the spinal cord and roof it over as with a 



