150 BRAINS OF RATS AND MEN 



sphere is very similar to that of the turtle, though 

 internally the structures are not so much alike. Com- 

 paring Figure 32 with Figure 20 (p. 104), the pyriform 

 lobe and amygdala are as large in the mammal as in 

 the reptile. In Orolestes (as in some reptiles) the cor- 

 pus striatum is entirely covered by pyriform cortex, 

 and so is the greater part of the amygdala. On the 

 medial surface the hippocampal cortex (Fig. 23i hip, 

 ant,) is apparently smaller than in the reptiles; but 

 this appearance is deceptive, for the greater part of 

 the elaborate hippocampus is rolled inward and con- 

 cealed from view. The pyriform cortex, too, is struc- 

 turally much more complicated than in reptiles, as 

 already illustrated (cf. Fig. 1 1 with Fig. 13, pp. 84, 85). 



The hippocampus and pyriform cortex are dom- 

 inated by the olfactory system. The non-olfactory, 

 or somatic, cortex forms the dorsal convexity of the 

 hemisphere, extending from the rhinal fissure lateral- 

 ly (Fig. 32, fs. rh.) to the hippocampal fissure medi- 

 ally (Fig. 33, fis, hip,). It is superficially scarcely 

 more extensive than in some reptiles, but structurally 

 it is much more complex (Fig. 14, p. 85). 



The olfactory cortex apparently attains almost its 

 maximum size and structural complexity in these very 

 primitive mammals, but the somatic cortex is here 

 smaller than in any other mammals with two or three 

 exceptions. The somatic cortex from this meager 

 beginning increases progressively in size and internal 

 structural differentiation up to its culmination m 



