THE ENCEPHALON. 



61 



fibres, the crura cerebri. Its roof, divided into two, or four, 

 convexities by a single longitudinal, or a crucial, depression, 

 is converted into the "optic lobes," corpcrra higemina or 

 quadrigemina. And these parts, the optic lobes, the crura 

 cerebri, and the interposed cavity, which either retains the 

 form of a ventricle, or is reduced to a mere canal (the 

 iter a tertio ad quartwm ventriculmn), are the components 

 of the mid-brain or mesencephalon. 



Fig. 20.— A longitudinal and vertical section of a Vertebrate brain. 

 The letters as before. The lamina terminalis is represented by the 

 strong black line between FM and 3. 



The anterior cerebral vesicle undergoes much greater 

 changes than either of the foregoing ; for, in the first place, 

 it throws out from its anterior lateral parietes two hollow 

 prolongations, the hemispheres (or frosencephala) , and each 

 of these again protrades from its anterior end a smaller 

 hollow process, the olfactory lobe (or rhinencephalon). By 

 the development of these processes the anterior vesicle 

 becomes divided into five parts — one median and posterior, 

 and four anterior and paired. The median and posterior, 

 which remains as the representative of the greater part of 

 the original anterior cerebral vesicle, is the vesicle of the 

 third ventricle (or thalamencephalon). Its floor is produced 

 into a conical process, the infundibulum, the blind end of 

 which is connected with the pituitary body, or hypophysis 

 cerebri. Its sides thicken greatly, acquire a ganglionic 



