132 THE BRAIN OF REPTILES 



the third ventricle. The two optic lobes are connected 

 with one another by a wide commissure, which constitutes 

 the roof of the above-mentioned passage. The optic 

 nerves arise from the under surface of these lobes. 

 They are lamellated structures ; and at the place where 

 the two nerves cross one another, their lamellae interlock ; 

 instead of the one nerve, as a whole, passing over the 

 other, as is the case in Fishes. 



In front of the optic lobes are the cerebral peduncles or 



* Crura Cerebri/ between which the * third ventricle ' is 

 situated. Stretching across this space, immediately in 

 front of the optic lobes, is the ' posterior commissure ' of 

 the brain, with wdiich (as in Reptiles) the peduncles of 

 the ' pineal body ' are connected — a structure sometimes 

 seen to project in the brain of Birds between the cerebral 

 hemispheres and the cerebellum. A little in front of this 



* posterior commissure ' a rounded prominence may be 

 seen on the upper and inner aspect of each cerebral 

 peduncle — that is, on the portion which constitutes part 

 of the lateral boundary of the third ventricle. A similar 

 projection has been previously alluded to as occurring 

 in some Reptiles, and it is supposed to correspond 

 with the important structures termed the ^Thalamus' 

 of a Mammal's brain. The anterior part of the floor of 

 the third ventricle still communicates, by a short hollow 

 peduncle, with the peculiar ' pituitary body '■ — a structure 

 which, in Birds#(tig. Gf), g) is proportionately less de- 

 veloped than in Reptiles and Fishes (fig. *30, g). 



The Cerebral Lobes are large and more or less rounded, 

 though they are flattened at their inner faces, where tney 

 come into contact with one another (fig. 65). These all- 

 important divisions of the brain are smooth and still 

 devoid of convolutions; yet in some birds thei'e are traces 

 of a depression, answering to a well-marked fissure (the 



