The Senses and the Nervous System 117 



of the brain, the cranial nerves, the nerve areas in the spinal 

 cord, the spinal nerves, and the sympathetic nerves. 



All we need to know is that basically the nervous system 



Shark 



Trout 



Figure 15. BRAINS 



In the three brains shown above, that of the dog is on a dijfferent scale 

 from the other two, which are in reality very much smaller. It is to be 

 noted that the cerebral hemispheres (CH), the parts of the brain used for 

 reasoning and imagination, are large and highly folded in the dog, rudi- 

 mentary in the shark and the trout. The olfactory regions (OL) are very 

 highly developed in the shark — in proportion even more than in the dog 

 — and are small in the trout. On the other hand, the optic lobes (O) are 

 comparatively tremendous in the trout} in the dog they are hidden beneath 

 the cerebral hemispheres. The medulla (M), the part of the brain where 

 simple responses originate, is largest in the shark and smallest in the dog. 

 The cerebellum (CB) is more important in the fish brains than in that of 

 the dog. It is obvious that the brain of the shark is dominated by the 

 olfactory area, that of the trout by the optic area, and that of the dog 

 by the reasoning area. (From Wiedersheim, Comfaratwe Anatomy of the 

 Vertebrates^ by permission of the Macmillan Company, publishers.) 



is the same as in higher animals, with one set of nerve-fibers 

 to carry sense-impressions from the sense-organs to the spinal 

 cord and the brain, another set of nerves to carry orders from 

 these centers out to the muscles, and a sympathetic system 

 to control the "automatic" workings of the internal organs. 



