THE BRAIN. 543 
as a specially differentiated part of the anterior region of the 
medullary groove and its subdivisions; and the close relation 
of the eye, ear, etc., to the brain in their early origin, is not 
without 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. 
Fig. 392. 
Fie. 393. 
Fia. 392.—Outer surface of human foetal brain at six months, showing origin of principal 
fissures (after Sharpey and R. Wagner). F, frontal lobe; P, parietal; O, occipital; T, 
temporal ; a, a, a, faint appearance of several frontal convolutions ; s, s, sylvian fissure ; 
s’, anterior division of same; C. central lobe of island of Reil; 1, fissure of Rolando; p, 
external perpendicular fissure. c 
Fic. 393.—Upper surface of brain represented in Fig. 090 (after Sharpey and R. Wagner). 
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- 
plexity of the human brain is referable to excessive growth of 
