BRAIN OF VERTEBRATA. 509 



the system of commissures, and which forms the posterior boundary 

 of the margin of the cleft, which always lies above the thalamen- 

 cephalon (cornu ammonis sive pes hippocampi major) (C it). 



The superior commissure is converted into two different, but 

 connected, structures. The sides of one bound the entrance into 

 the lateral ventricles superiorly, and it passes, at its sides and below, 

 into a band which is placed on the hippocampus major. This portion 

 (fornix) (B Of) commences anteriorly by ascending columns, is 

 somewhat broadened out over the thalamencephalon, and is con- 

 tinued into the posterior descending columns. Superiorly it is 

 connected with a portion of the commissural system, the corpus 

 callosum, which, at first continuous with it, becomes anteriorly 

 separated from it. The backward extension of these commis- 

 sures is dependent on the development of the hemispheres of the 

 prosencephalon, which are feebly developed in Rodents, Edentates, 

 and Insectivora. According as they increase in size, the anterior 

 commissure is diminished in area. In the Monotremata and Didel- 

 phia it is very much so, and is converted into a thin chord placed 

 in front of the columns of the fornix. The more the hemispheres 

 of the prosencephalon are enlarged backwards, the more do they 

 overlie the parts behind them. 



In many Mammals the hemispheres are smooth, and the surface 

 of the prosencephalon is then in a simple stage, corresponding to 

 its embryonic characters ; these may be complicated by gyri and 

 sulci. The gyri are at first arranged regularly and symmetrically, 

 and only become asymmetrical when they are more largely developed, 

 as they are, for instance, in Man. But even in this case they may 

 be divided into groups, the boundaries of which are formed by the 

 earliest, which are in some Mammals the only, sulci present. The 

 gyri are similar in character in their earliest stages only. When 

 they are complicated, the arrangements differ in different divisions 

 of the Mammalia, and serve as indications of the degree of affinity 

 that there is between such divisions. 



The thalamencephalon is divided into two masses, which lie 

 immediately behind the corpora striata of the lateral ventricles of the 

 prosencephalon, the thalami optici. The epiphysis is placed at the 

 hinder end of the cleft which separates them. The cavity of this 

 portion is reduced to a small space between the "two thalami, and is 

 continued downwards into the inf undibulum, which is carried by the 

 tuber cinereum. 



The mesencephalon, which for a long time forms the largest 

 division of the brain (Fig. 280, c), has its primitive lumen gradually 

 converted into a narrow canal (aqueductus Sylvii), which unites the 

 third with the fourth ventricle. Its surface is divided by shallow 

 longitudinal and transverse grooves into four bodies (Fig. 286, B 

 III), in consequence of which the corpus bigeminum becomes the 

 corpora quadrigemina. This division into four lobes is very slight in 

 the Monotremata. 



The metencephalon (cerebellum) only resembles that of the 



