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 which (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 (fig. 66, 6) is proportionately less de- 

 veloped than in Reptiles and Fishes (fig. 60, o). 



The Cerebral Lobes are large and more or less rounded, 

 though they are flattened at their inner faces, where they 

 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 there are traces 

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



