THE BEAIN. 



511 



medullary groove and its subdivisions ; and the close relation 

 of the eye, ear, etc., to the brain in their early origin, is not 

 witTiout special meaning, while the more diffused sensory de- 

 velopments in the skin connect the higher animals closely with 

 the lower — even the lowest, in which sensation is almost wholly 

 referable to the surface of the body. 



Without some knowledge of the mode of development of 

 the encephalon, it is scarcely possible to appreciate that rising 

 grade of complexity met with as we pass from lower to higher 

 groups of animals, especially noticeable in vertebrates ; nor is 

 it possible to recognize fully the evidence found in the nervous 

 system for the doctrine that higher are derived from lower 

 forms by a process of evolution. 



Evolution. — The same law applies to the nervous system as 

 to other parts of the organism, viz., that the individual devel- 

 opment (ontogeny) is a synoptical representation, in a general 

 way, of the development of the group (phylogeny). A com- 

 parison of the development of even man's brain reveals the fact 

 that, in its earliest stage, it is scarcely, if at all, distinguishable 

 from that of any of the lower vertebrates. There is a period 

 when even this, the most convoluted of all brains, is as smooth 

 and devoid of gyri as the brain of a frog. The extreme com- 



FiG. 367.— A, brain of aye-aye {Lemur); B, of marmoBet; O, of sqnirrel monkey [Cai- 

 nthrix); D, of macaque monkey; E, of gibbon; F, of a flfth-montli linman foetus 

 (after Owen). Although nataraliets are agreed that the monkeys, apes, and lemurs 

 are related, considerable differences are to be observed in their brams. These fig- 

 ures also illustrate the remark made after those following. 



plexity of the human brain is referable to excessive growth of 

 certain parts, crowding and alteration of shape, owing to the 

 influence of its bony case, its membranes, etc. 



