ANATOMY, PHYSIOLOGY, OF NERVOUS SYSTEM. 419 



upwards towards the top of the brain separating from the 

 main portion of the cerebrum, the frontal lobes, is the 

 fissure of Rolando. The fissure of Sylvius is important as 

 being the most apparent and deepest furrow. The fissure 

 of Rolando is interesting as being the region along which 

 many of the centers of conscious volition have been by ex- 

 periments localized. If the fissure of Sylvius be opened 

 with the fingers it discloses to view a lobe of the brain 

 hidden in this fissure known as the Island of Rcil. If from 

 the top the two cerebral hemispheres be pushed apart it will 

 be seen that the median fissure reaches down to a white 

 band of connecting fibers which runs from one hemisphere 

 to the other. This band of connecting fibers is called the 

 corpus callosum. 



Base of Drain. If now the base of the cerebrum be 

 studied it reveals a number of structures easily recognizable 

 with the unaided eye. Lying immediately under the frontal 

 lobes are the olfactory lobes. These are bits of grayish, 

 nervous tissue and are the nerves concerned in the sense of 

 smell. They are quite inconspicuous in man, but in some 

 of the lower animals reach very large proportions, some- 

 times being larger than the cerebral hemispheres themselves. 

 This may probably be interpreted as meaning that the sense 

 of smell is relatively dull in man as compared with many of 

 the lower forms, which have to rely upon this sense in 

 searching for their food or avoiding their enemies. 



Immediately back of the olfactory lobes the large optic 

 nerves arise. These optic nerves seem to cross at the base 

 of the brain in the optic commissure and are continued back 

 into the brain beyond this commissure as the optic tracts. 

 These tracts may be followed some distance around the 

 crura cerebri into the tissue of the brain. There is not a 

 complete crossing, however, of the optic nerves at the com- 

 missure, but the decussation is limited to half of the fibers 

 so that the optic nerve on each side consists of half of the 

 fibers from its own optic tract, the other half from the op- 

 posite optic tract which reached it in the commissure. 



