BKAIN AND NERVE 



95 



cavities persist all through life as the ventricles, which are in 

 open communication with the tubular cavity, or central canal, 

 of the spinal cord. 



Next the walls of the brain vesicles begin to thicken very 

 unequally, and bending occurs. Thus, in the human embryo 

 at about the 



middle of the ^^"^ ^'"^ ^'"'^ 

 eighth week 



we find the 3ra.in 



brain some- 

 what as given 

 in Fig. 25. 



Fig. 25 re- 

 presents the 

 stage through 

 which all ver- 

 tebrate ani- 

 mals pass in 

 the course of 

 their embryo- 

 geny. The 

 floor parts of 

 the lateral ves- 

 icles thicken to 

 form the cor- 

 pora striata ; 

 the sides of 

 the fore-ves- 

 icle similarly 

 thicken, to be- 

 come later on 



3 rain 

 vesicles 



Ldtera I 

 Fore -' 

 NidL-- 

 Hind- 



^pina.1 cord. 



^ - ;::r Corpora striata. 



r'-:Obtic vesicles 



_^^ -'Mid- brain 



•-' (CorborcL . » 



ooaari£em/na j 



''^Cerebellum 

 --J^edulla. 



spinal Cord 



Fig. 24.— Development op the Beain. 



The upper figure shows the primitive brain tube and 

 its vesicles. The lower figure shows further stages. 



the optic thalami (or the parts in lower vertebrates corresponding 

 to the human thalami); the roof of the mid- vesicle forms the 

 corpora quadrigemina of man, or the lower vertebrate homologues 

 of these ganglia; the roof of the front part of the hind- vesicle 

 becomes the cerebellum ; and the floor and sides of the posterior 

 part become the medulla. The cerebellum is very small in fishes, 

 and becomes more important as we rise in the scale, until it 

 practically becomes a new part added to the primitive brain. 

 The roofs of the lateral vesicles are never more than mere mem- 

 branes, non-nervous structures, in fishes, but they thicken in 



