972 , EVOLUTIONAL MODIFICATIONS 



In the case of the optic ncr\c, a similar record of expansion is to he found. 

 This is seen in the degrees of angulation in the conAergence ol the optic 

 nerves upon the chiasm and the dixergence ol the optic tracts Irom it. 

 This angulation gradually increases in passing from the lennir, through the 

 intermediate ])rimates, to the great anthropoids, and reaches its greatest 

 prominence in man. It is due to the fact that expansions in the head have 

 further sej^arated the eyes from each other. The expansions themselves are 

 due in large measure to the growth of the brain areas abo\e the orbits. 

 Similar txpaiisions in portions of the brain caudal to the optic chiasm arc in 

 process and account for the wider angle of divergence in the optic tracts. 

 This feature w hile far less significant than olfactory dt'xelopnu'nt possesses 

 a value, which, however slight, should carr\ its due weight in estimating the 

 evolution of the bram. 



Evolutional Significance of Cerebellar Expansion in the Brain. 

 The inllucnces of expansion may bi' further discerned m the occipital 

 lobe. This extension likewise affects the lateral lobes of the cerebellum. In 

 the lowest of the primates the cerebellar conea\ it\ is a deep impression made 

 upon the basal surface of the occipital lobe In' the cerebellum. It is less pro- 

 nounced in lenuir than in higher species because the cerelx-llum is onl\ m 

 part coxered by the occipital lobe. In the marmoset and Hapalidae generally 

 this concaxity becomes more prominent, due to the lact that the cerebellum 

 indents the under surface of the brain to a considerable degree, while the 

 occipital loi)e entirt'ly eo\ers the tentorial surface of the cerebellum. The 

 cerebellar conca\ity, and more [)art ieularl\ the postsplenial fossa, is a 

 character obserxed in all primate brains. Tlu' lateral e\])ansions ol it bt'come 

 progressively less jjronounced in the inti'rmedlate ]:)rnnates. It is well cK'Imed 

 in the orang and ihimpanzee, but becomes less [jromment m the gnnlla and 

 almost entirel\ disappears in man where it leaxcs but a small Acstige ol the 

 postsplenial fossa to accommodate tlu- \Ainiis. 



