REPTILIA 



389 



reduced or even altogether aborted. This is an adaptation to 

 the narrow elongated body cavity. The ribs when present and 

 the muscles acting on them are the prime agents in breathing. 



419. Circulation. — In reptiles the right and left auricles are 

 entirely distinct but, with the exception of the Crocodilia, the 

 ventricles are only partially so. The septum in the ventricle is 

 perforated. Yet in those forms in which the pure blood of the 

 left auricle and the impure blood of the right partially mingle 

 in the ventricle, the arrangement is such that the purest blood 



Fig. 195. 



pt. 2, 



Fig. Z95. Vertebrae of a Reptile (after Huxley). A, anterior view; B, posterior view of the 

 vertebra in front of A. The surface of A fits against the surface of B. c, centrum, which is convex 

 in B, fitting into a concavity in A; n.s., neural spine; pr.z., pre-zygapophyses, or anterior articular 

 facets, which fit against pt.z., post-zygapophyses; t^p,, transverse processes; E.5.. a wedge-like 

 articular face on the neural arch designed to fit into s.a., a depression on the posterior face of the 

 neural arch o' the vertebra in front (B). 



Questions on the figures. — Try to form a clear picture of the relations of the 

 articulating surfaces of the vertebrae and indicate the possible advantages of the 

 arrangements. Where is the neural cavity? Where do the ribs articulate? 

 What is the gain in muscular attachments from the numerous bony outgrowths 

 on the vertebrae? 



goes to the brain and the least pure to the lungs (see Fig. 170). 

 Two aortic arches unite, giving rise to the dorsal aorta. In the 

 reptiles and higher groups of vertebrates the renal-portal circu- 

 lation (see Fig. 181, r.p.) ceases to be of much importance, but 

 the hepatic portal is increasingly important. The red corpuscles 

 are elliptical and possess nuclei. 



420. Nervous System and Special Sense Organs. — The 

 brain is not l3.rge in the reptiles, but is rather more highly de- 



