NERVOUS SYSTEM 167 



and in man they pass singly through the Lamina cribrosa. In 

 the Cyclostomata, on the contrary, each optic nerve passes 

 separately to the eye subsequent to the formation of a com- 

 missure near the point of origin, while in the other Vertebrates 

 an optic chiasma is always present. It must moreover be re- 

 marked that only in the reptiles, birds, mammals and man do 

 the oculo-motor, trochlear and abducens nerves leave the brain 

 separately and remain separate throughout their entire course. 

 The vagus, which appears in the reptiles (snakes excepted), 

 is closely connected with the accessories only in the higher 

 Vertebrates and in man. The rest of the cerebral nerves present 

 no points of difference among the various classes of Vertebrates. 



Having discussed the separate parts of the brain from the 

 standpoint of comparative anatomy, it remains only to examine 

 the entire mass of the human brain as compared with separate 

 parts of the body, and with the weight of the entire body. 



On the authority of Vierordt l the absolute weight of the 

 brain of an individual of twenty to thirty years is : 



In the German man 1,461 gm. ; woman 1,341 gm. 



English 1,412 1,292 



French 1,358 1,230 



Swiss 1,350 1,250 



Russian 1,346 1,195 



Davis has drawn up tables according to the races. He gives 



as the average absolute weight of the brain : 



European man 1,367 gm. ; woman i,2O4gm. 

 Oceanian .1,319 I , 2I 9 



American 1,308 1,187 



Asiatic 1,304 1,194 



African 1,293 1,211 



Australian ,, 1,214 ,, 1,111 



The figures show that the male brain is on an average 9 per 

 cent, heavier than the female. 



The increase of size in the human brain due to its convolu- 

 tions (as contrasted with a smooth, unconvoluted brain) Dumou- 

 lins reckons as twelve-fold. 



1 H. Vierordt, Anatomische, physiologische und physikal. Daten und Tabellen. 

 Jena, 1888. P. 39. 



