EXTEKNAL EEATUKES OP THE HUMAN BKAIN 393 



than the frog's has, and this organ is here divided into two 

 lobes by a longitudinal partition. Underneath the cere- 

 bellum, and extending downward, is the wedge-shaped 

 medulla. In comparison with our figure the hindbrain 

 occupies the position of the stem of the interrogation point, 

 and if this were continued downward the extension would 

 represent the spinal cord. 



External features of the human brain. — Viewed from 

 the top, the two hemispheres of the forebrain (the cere- 

 hrum) are seen to be much convoluted and folded, thus 

 increasing their surface without increasing the space occu- 

 pied. The two hemispheres are pressed closely together, 

 the line of separation 

 being marked by a fissure 

 (the longitudinal fissure). 

 Viewed from the bottom 

 we can make out the con- 

 tinuation of this fissm-e 

 back to the midbrain, and 

 on each side of it appear 

 the olfactory lobes. (See 

 Fig. 182.) Just back of 

 these lobes are the crossed 

 optic nerves (the optic 

 chiasma) which enter the 

 optic lobes. Back of this 

 chiasma is a band of white 

 matter (the pons varolii) 



which connects the two lobes of the cerebellum and 

 covers the top of the medulla. Underneath this pons, 

 the medulla divides into two stalks (the crura cerebri) 

 which spread out to form the cerebral hemispheres, 



Fig. 181- 



Cerebral hemispheres viewed 

 from above. 



