THE NERVOUS SYSTEM 



291 



free from all its coverings and looked at from above, little is 

 seen but two large hemispherical masses, separated by a deep 

 fissure. These masses make up the cerebrum, and are called 

 the cerebral hemispheres. If the brain is tilted forward so 

 that the back part of the cerebrum is visible, there comes into 

 view the cerebellum, which also shows an open groove be- 

 tween its right and 



left halves. Below Cerebrum 



the cerebellum is the 

 medulla, which ex- 

 tends downward and 

 passes, without any 

 special line of separa- 

 tion, into the spinal 

 cord. These parts 

 are shown in side 

 view in Figure 146. 

 Viewed from below 

 (Fig. 147), the same 

 structures can be 

 recognized, and the 

 olfactory or smelling 

 nerves should also 

 be noticed under the 



front lobes of the cerebrum. Going from front to back, 

 observe next between the two cerebral hemispheres the 

 large optic nerves going to the eyes, the nerve from the 

 right side, and the one from the left meeting in the middle 

 line. The place where these fibres mingle forms an X-like 

 structure, called the optic chiasma, just beneath which is 

 the small, round pituitary body, whose function is not ex- 

 actly known. Behind this, notice on each side a length- 

 wise ridge. These two ridges, which are called the crura 

 cerebri, converge backward until they become the right 

 and left halves of the spinal cord. Back of the crura 



Cerebellum 



FIG. 146. THE HUMAN BRAIN 



Shown from the side with the cerebrum and cerebellum 

 separated from each other 



