CHAPTER XXV. 



THE NERVOUS SYSTEM AND SPECIAL 

 FUNCTIONS. 



As in nearly all vertebrates, the nervous system may 

 be divided into the cerebro-spinal and the sympathetic 

 system, with the brain as the front dilation of the nerve 

 axis. Beginning with the lowest vertebrate animals with 

 a nervous axis, the front end of the nerve cord is practically 

 without dilation, that is, there are practically no evident 

 lobes, such as characterize the human brain, for example. 

 In fishes, there are the characteristic lobes, front and rear 

 brain, and olfactory and optic lobes, but of nearly equal 

 size, the optic lobes being somewhat the largest of the 

 four. In the reptilian brain the lobes are of more un- 

 equal size, owing to the encroachment of the front brain 

 upon the cavity occupied by the other lobes of the front 

 end of this nerve axis. In birds the front brain partly 

 covers the olfactory and the optic lobes, and is relatively 

 larger than it is in reptiles. That is, the cerebral hemi- 

 spheres are larger than in any of the animals below the 

 birds. This relative proportion of the four lobes is one 

 of the bases of determining the place of an animal in the 

 scale of life. Of course, there must also enter the propor- 

 tion of gray matter to white, and the fineness of the matter 

 making up the nervous tract, as well as the comparative 

 size of the four lobes. 



There are the familiar twelve pairs of cranial nerves, 

 and, as in man, the number of spinal nerves corresponds 



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