360 



ZOOLOGY 



SECT. 



aorta. But in the Crocodilia (Fig. 1003) the cavity is com- 

 pletely divided, so that there we may speak of distinct right 

 and left ventricles. From the right arises the pulmonary artery 

 and the left aortic arch : from the left the right aortic arch 



only. The right and left arches 

 cross one other, and where their 

 walls are in contact is an 

 aperture the foramen Panizzce 

 placing their cavities in com- 

 munication. 



The brain of Reptiles is some- 

 what more highly organised than 

 that of the Amphibia. The brain 

 substance exhibits a distinction 

 into superficial grey layer or cor- 

 tex containing pyramidal nerve- 

 cells, and central white medulla, 

 not observable in lower groups. 

 The cerebral hemispheres are 

 well developed in all, and there is 

 a hippocampus (see below in the 

 description of the brain of the 

 Rabbit, and of that of the Mam- 

 mals in general) in the shape of 

 a specially modified region of the 

 dorsal and mesial walls of each 

 hemisphere, represented less dis- 

 tinctly in the Amphibia ; a com- 

 missure the hippocampal con- 

 nects the hippocampi of opposite 

 sides, and is dorsal to the chief 

 cerebral commissure the an- 

 terior commissure. The mid-brain 

 consists usually of two closely- 

 approximated oval optic lobes ; 

 rarely it is divided superficially 

 into four. The cerebellum is 

 always of small size, except in 

 the Crocodilia (Fig. 1004), in 

 which it is comparatively highly 

 developed, and consists of a 

 median and two lateral lobes. 



Sensory Organs. In most Lacertilia, but not in the Ophidia, 

 the nasal cavity consists of two parts an outer or vestibule, and 

 an inner or olfactory chamber the latter having the sense-cells in 

 its walls, and containing a turbinal bone. In the Turtles each 

 nasal chamber is divided into two passages, an upper and a lower, 



Med 



FIG. 1004. Brain of Alligator, from 

 above. B. ol. olfactory bulb ; G. p, epi- 

 physis ; HH, cerebellum ; Med, spinal 

 cord ; MH, optic lobes ; NH, medulla 

 oblongata ; VH, cerebral hemispheres ; 

 / XI, cerebral nerves; ], 2, first and 

 second spinal nerves. (From Wieder- 

 shcim's Comparative Anatomy.) 



