n8 THE COMMON FROG. [chap. 



man, it is at first sight difficult to find out the resem- 

 blances, and to determine which portions of the one 

 answer to definite regions of the other. (Fig. 74.) 



Fig. 72. - Enlarged and Diagrammatic View of a Vertical Section carried through 

 the Corpus Callosum (of Man) and the parts below, ac, anterior commissure ; cc, 

 corpus callosum ; cbl, cerebellum; cm, corpus mammillare; y, fornix ; Jw, foramen 

 of Monro ; r, infundibulum ; Ip, locus perforatus medius ; mo, medulla oblongata ; 

 na, nates ; on, optic nerve ; pc, posterioB- commissure ; pv, pons Varolii ; pi, pineal 

 gland ; pt, pituitary body ; s, soft, or middle commissure ; si, septum lucidurn ; 

 t, lamina terminalis ; te, testes ; v, velum interpositum (between it and the fornix 

 is a space enclosed by the folding over of the cerebrum upon the roof of th« 

 third ventricle) ; 3, upper, and 3" lower part of the third ventricle ; 4, fourth 

 ventricle — between them is the iter a tertio ad quartujii ventriciibim. 



In the earliest conditions of the human brain the 

 resemblance is much more marked and obvious ; it is 

 later that the correspondence between the brain of the 

 frog and that of man becomes so disguised through 

 the unequal growth of different portions of the organ 

 in the human brain as it advances in its growth and 

 development. The same six successive portions, how- 

 ever, exist in each. 



1. In the frog the olfactory lobes acquire a much 

 larger relative size, and they retain permanently an 

 internal cavity which exists only transitorily in man. 



2. The cerebral lobes (or hemispheres) exceed those 

 just noticed, but are insignificant indeed, when com- 

 pared with the corresponding human structures. They 



