THE BRAIN 



157 



In Fishes and Amphibians the cerebral flexure later becomes 

 practically obliterated, but it persists more or less markedly in the 

 higher types, more particularly in Mammals. In the latter Class, 

 moreover, the original relation of the parts becomes still further 

 complicated by *the large development of the cerebral hemispheres, 

 which grow backwards, and thus gradually come to overlie all the 

 other parts of the brain. This condition of things attains its 

 greatest perfection in Man. Thus instead of 

 the various sections of the brain being situated 

 one behind another, they come to lie eventually. 

 more upon one another, the thalamencephalon, 

 mid-brain, cerebellum, and medulla oblongata 

 becoming covered over by the hemispheres. 



Amphioxus. — The conical and enlarged anterior 

 end of the spinal cord of the Lancelet contains a 

 widened portion of the central canal which must be 

 looked upon as a ventricle. In the larva, this opens 

 freely on to the exterior dorsally by a neiiropore, 

 which probably represents the last indication of the 

 primitive connection of the central nervous system 

 with the outer skin. It is possible that the anterior 

 enlargement of the cord corresponds to the fore- 

 brain — and perhaps also the mid-brain — of tlie 

 Craniata. 



Cyclostomi. — The brain of these forms 

 remains in many respects in an embryonic con- 

 dition : this is particularly the case in the larval 

 PetromyzonorAmmocoete(Fig.l29). Intheadult 

 the individual vesicles lie in an almost horizontal 

 direction one behind the other, and the prosen- 

 cephalon consists of a median part and of small 

 paired hemispheres continuous anteriorly with 

 the larger, rounded olfactory lobes. The median 

 portion of the prosocoele is continued trans- 

 versely outwards into each hemisphere, in which 

 it gives rise to a lateral ventricle : this is con- 

 tinued forwards for a short distance into the 

 base of the olfactory lobe, as well as backwards 

 into the hemisphere. The roof (pallium) of 

 the median portion of the ventricle is non- 

 nervous, and consists of a sivgle layer of epithelial cells, which, 

 together with the pia mater, has been removed in the prepa- 

 ration represented in Fig. 129, A. The mid-brain and medulla 

 oblongata are relatively broad, and the cerebellum is represented 

 by a mere narrow ledge overhanging the fourth ventricle ante- 

 riorly. The roof of the mesocoele is formed mainly by a layer of 

 epithelial cells, and, like that of the third and fourth ventricles, is 



Fir 128.— Cerebral 

 Flexure of a 

 Mammal. 



VH, prosencephalon; 

 ZH, thalamence- 

 phalon, with the 

 pituitary body 

 (H) at its base ; 

 MH, mesenceph- 

 alon, which at SB 

 forms the most 

 projecting por- 

 tion of the brain, 

 representing the 

 so-called " parie- 

 tal bend " ; HH, 

 metencephalon ; 

 NH, myelence- 

 phalon, forming 

 the " cervical 

 bend" (iV.S) : the 

 "Varolian bend" 

 {BB) arises on 

 the ventral cir- 

 cumference, at 

 the junction be- 

 tween HH and 

 XH; R, spinal 

 cord. 



