506 



COMPARATIVE ANATOMY. 



I 



§ 381. 



The brain of the Amphibia resembles that of the Fishes in many 

 points. The prosencephalon (Fig. 283, h) is divided into two hemi- 

 spheres, and shows signs of being enlarged backwards. The cavity 

 within it is divided into two lateral ventricles, one for each half, and 

 these are continued forwards into the olfactory lobes (a). At first, 

 these lobes are placed at the side of the prosencephalon (h), and 



are directly attached to it, but they may 

 become closely fused with the prosence- 

 phalon, and with one another. The olfactory 

 nerve arises on their lower surface, some way 

 back, and near the prosencephalon. The 

 thalamencephalon is differentiated during 

 the larval stage from a portion which is 

 common to it and the mesencephalon. In 

 front of it is the primitive cerebral cleft, 

 which is more or less continued into the 

 thalamencephalon, and carries the epiphysis 

 cerebri. The cleft extends anteriorly into 

 the lateral ventricles, which are enclosed 

 by the two hemispheres of the prosence- 

 phalon. On the lower surface of this por- 

 tion there is an eminence, which corresponds 

 to the lobi inferiores. 



In the Urodela the mesencephalon re- 

 mains at a certain stage, which is temporarily 

 presented by the Anura; it is in the latteronly 

 that it becomes of any size, and is divided 

 into two halves (c) . The metencephalon, how- 

 ever, retains its primitive form of a lamella, 

 which bridges over the fourth ventricle {d). 

 In the brain of the Reptilia the angula- 

 tion which we already observed in Fishes has 

 been much increased in the region of the 

 thalam- and mes-encephalon, owing to the 

 increased development of the upper parts ; 

 this produces a change in the relative position of the parts, which 

 is still more marked in the higher divisions (compare the sections 

 in Fig. 280). The prosencephalon is more largely developed, and has 

 the form of two hemispheres, covering the thalamencephalon, and 

 broadest behind. The olfactory lobes are attached directly to them. 

 The lateral ventricles are very large, and communicate at the cerebral 

 cleft with the third ventricle, which is placed between the two 

 halves of the thalamencephalon, and is provided with a large in- 

 fundibulum. The mesencephalon is divided by a groove into two 

 hemispheres, which sometimes project very far forwards. The 

 metencephalon varies a great deal ; in the Ophidii and Saurii it 



Fig. 283. Brain aud spinal 

 chord of the Frog. A from 

 above, B from below. 

 a Lobi olfactorii. h Prosen- 

 cephalon, c Mesencephalon, 

 d Metencephalon. eMyelen- 

 cephalon. s Fourth ven- 

 tricle, in Spinal chord. 

 t Filum terminale of the 

 chord. 



