58 THE ANATOMY OF VERTEBRATED AXIMALS. 



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

 it throws out from its anterior lateral parietes two hollow 

 prolongations, the hemispheres (or prosencephala)^ and each 

 of these again protrudes from its anterior end a smaller 

 hollow process, the olfactory lohe (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 infundibulwn^ the blind end of which is 

 connected with the pituitary body, or hypophysis cerebri. Its 

 sides thicken greatly, acquire a ganglionic structure, and 

 become the optic thalarni. Its roof, on the other hand, 

 resembles that of the fourth ventricle, in remaining very thin, 

 and, indeed, a mere membrane. The pineal gland, or epiphy- 

 sis cerebri^ is developed in connection with the upper wall of 

 the third ventricle ; and, at the sides of its roof, are two ner- 

 vous bands, which run to the pineal gland, and are called its 

 peduncles. 



The front wall of the vesicle, in part, becomes the so-called 

 lamina terminalis, which is the delicate anterior boundary of 

 the third ventricle. In certain directions, however, it thickens 

 and gives rise to three sets of fibres, one transverse and two 

 vertical — the former lying in front of the latter. The trans- 

 verse fibres pass on either side into the corpora striata, and 

 constitute the anterior commissure which connects those bodies. 

 The vertical fibres are the anterior pillars of the fornix, and 

 they pass below into the floor of the third ventricle, and 

 into the corpora mammillaria, when those structures are de- 

 veloped. 



The outer and under wall of each cerebral hemisphere 

 thickens and becomes the corpus striatum^ a ganglionic struct- 

 ure which, from its origin, necessarily abuts against the outer 

 and interior part of the oj^tic thalamus. The line of demar- 

 cation between the two corresponds with the lower lip {taenia 

 semicircularis) of the aperture of communication (called the 

 foramen of 3Iunro) between the third ventricle and the 

 cavity of the cerebral hemisphere, which is now termed the 

 lateral ventricle. In the higher Vertebrata, the upper lip of 

 the foramen of Munro thickens, and becomes converted into a 

 bundle of longitudinal fibres, which is continuous, anteriorly, 

 with the anterior pillars ot the fornix before mentioned. Pos- 



