290 THE HUMAN BODY. 



rated so much as has been represented in the diagram for 

 the sake of clearness, but lie close together as represented 

 in Fig. 83; and the mid-brain is entirely covered in on its 

 dorsal side. Nearly everywhere the surface of the 

 brain is folded, the folds, known as the convolutions, 

 being deeper and more numerous in the brain of man 

 than in that of the animals nearest allied to him zoologi- 

 cally. 



The brain, like the spinal cord, consists of gray and 

 white nervous matter, but somewhat differently arranged; 

 for while the brain, like the cord, contains gray nerve-mat- 

 ter in its interior, a great part of its surface is also covered 

 with it. By the external convolutions of the cerebellum 

 and of the cerebral hemispheres the surface over which this 

 gray substance is spread is very much increased. 



The Cranial Nerves. Twelve pairs of nerves leave the 

 skull cavity by apertures in its base; they are known as the 

 cranial nerves. Most of them spring from the under side 

 of the brain, which is represented in Fig. 84. The first 

 pair, or olfactory nerves, are the nerves of smell; they arise 

 from the under sides of the olfactory lobes, /, and pass out 

 through the roof of the nose. The second pair, or optic 

 nerves, II, are the nerves of sight; they spring from the 

 mid-brain, and, under the name of the optic tracts, run 

 down to the under side of the fore-brain, where they unite 



Is the surface of most of the brain smooth? What are the folds 

 called? How does a man's brain differ, as regards its convolutions, 

 from an ape's? 



Of what does the brain consist? How does the arrangement of 

 white and gray matter in it differ from that of the spinal cord? How 

 is the surface on which the gray matter is spread increased ? 



How many cranial nerves are there? Where do most of them 

 originate? Name the first pair. Where do they arise? Where do 

 they pass out ? Name the second pair. What are the optic tracts? 



