72 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



COMPARATIVE MORPHOLOGY AND PHYSIOLOGY OF THE 

 VERTEBRATE NERVOUS SYSTEM. 



The general plan of the nervous system is so pre- 

 cisely the same in all vertebrates that only the most 

 general statements are necessary in regard to this. In 

 all vertebrates, but in no other animals, we have both 

 an axial and a ganglionic system. In all vertebrates 

 the axial system consists of a continuous tract of gray 

 matter inclosed in white matter lying along the dorsal 

 aspect of the body, enlarged at the anterior end to form 

 a brain, and giving off nerves in pairs from one end to 

 the other (Fig. 14, page 29). In different vertebrates 

 the number of these pairs vary, being least in frogs and 

 toads, where there are only eighteen or twenty, and 

 greatest in some fishes, as the eels, where they may be 

 two hundred or more. Therefore the only part where 

 the differences are important enough to arrest our at- 

 tention in this rapid sketch is the brain. 



THE BRAIN OF VERTEBRATES. 



In running down the vertebrate scale there are three 

 important changes which take place in the brain : 1. In 

 size, both absolute and relative. 2. In relative amount 

 of gray matter compared with white, as shown by the 

 complexity of the convolutions. 3. In the relative size 

 of the cerebrum as compared with the other ganglia of 

 the brain. Perhaps I may add : 4. In the relative size of 

 the frontal lobe compared with the other lobes of the 

 cerebrum, as shown by the position of the fissure of 

 Rolando. In all these respects the brain of man stands 

 pre-eminent. 



1. Size {a) Absolute. — The brain of man weighs about 

 three pounds (forty-eight to fifty ounces). The heaviest 

 which have been weighed — viz., that of Cuvier, the great 



