REPTILES. 



295 



posterior dorsal portion or the right lung is supplied with blood 

 from the dorsal aorta. In the chameleons and geckoes the 

 lungs give off large saccular projections, recalling the air sacs 

 so characteristic of birds. In some dinosaurs the bones exhibit 

 a marked pneumaticity, and it is supposed that in these the air 

 sacs penetrated the bones. In the snakes the chambering of 

 the lung is restricted to the peripheral portion, the centre being 

 occupied by a large air space, and about the same conditions 

 occur in most lizards. In the chameleons, however, each bron- 

 chus, on entering the lungs, divides into three parts, and the 

 proximal portion of the lung is sacculated, while distally all three 

 bronchi connect with a common space, without alveoli. In 

 crocodiles and chelonians the sub-division of the lungs is carried 

 farther. 



The brain presents advances in several points upon the con- 

 ditions in the amphibia. Thus there is , here developed a cere- 

 bral cortex of gray matter containing pyramidal 

 cells. The cerebrum exhibits a tendency to ex- 

 tend backwards, covering in the thalamencepha- 

 lon. The olfactory lobes may be seated directly 

 on the cerebrum, or an elongate olfactory tract 

 may intervene. The olfactory fibres do not ex- 

 tend back to the corpus striatum, but a distinct 

 olfactory centre is developed in the pallium. 

 Hippocampal lobes occur in a few forms (^Hat- 

 teria, crocodiles, chelonians). The twixt brain 

 is at a lower level than the rest, the infundib- 

 ular region being well developed. The mid 

 brain is large, and its two halves rarely exhibit 

 a tendency towards division into four. In the 

 cerebellum there is a great range of structure, 

 from forms in which it is merely a transverse 

 fold, up to the crocodiles, where it consists of 

 two lateral lobes and a median portion, recall- 

 ing the vermis of the mammals. In the medulla 

 occurs the characteristic nuchal flexure. 



In the cranial nerves the marked feature is the distinct ori- 

 gin of nerves, the roots of which are closely approximate in the 



Fig . 293. 

 Brain of garter- 

 snake, Eutainia 

 sirtnlis. 



