THE LIFE PROCESS IN VERTEBRATES. 263 



IV. Sensation. The position of the central nervous 

 system (i.e., of the brain and spinal cord) in vertebrates, 

 dorsal to the center of the bony axis of the skeleton, and 

 inclosed within a dorsal, neural canal, is very charac- 

 teristic of the group. In the lowest, the brain is but 

 little developed at the anterior end of the spinal cord; 

 but, as we ascend the vertebrate scale, it becomes more 

 and more highly specialized. The cerebral hemispheres, 

 which are the seat of intelligence, reach their relative 

 maximum development in man. The spinal and cranial 

 nerves are distributed to all the muscles of the body, to 

 the skin, and certain of them to the blood vessels and 

 to the internal organs ; and all these parts are thus placed 

 in communication with the nerve centers. A double 

 series of sympathetic ganglia extends along the dorsal 

 wall of the body cavity, and sends nerves to the digestive 

 and reproductive organs. 



The five senses of man are probably possessed in vary- 

 ing degrees by all other vertebrates, and each of the 

 five is probably keener in some other animal than in 

 man. 



Of the higher possibilities of the vertebrate brain it is 

 impossible to speak here. Volumes would be required to 

 adequately summarize the wonderful and infinitely varied 

 instincts of fishes, batrachians, reptiles, birds, and mam- 

 mals. A few of these have been pointed out in the cases 

 of the types studied : others invite study in every locality, 

 on every hand. Such study will not be amiss if it lead 

 to the study of the instincts that move men. 1 It will be 

 especially valuable if the student see, that in proportion 

 as man learns to govern his instinctive impulses, and be- 

 comes a creature of ideals, he ascends above the level of 

 the brute. 



1 For a good discussion of this subject, the student is referred to the 

 closing chapters of Chadbourne's Instinct in Animals and in Man. 



