NERVOUS SYSTEM AND GENERAL SENSATION. 



21 



part characteristic of the animal body. It is the grand cen- 

 tre from which all the commands of the will issue, and to 

 which all sensations tend. 



61. Greatly as the form, the arrangement, and the vol- 



ume of the nervous system 

 vary in different animals, 

 they may all be reduced to 

 four principal types, which 

 correspond moreover, to the 

 four great departments of the 

 Animal Kingdom. In the 

 vertebrate animals, namely, 

 the fishes, reptiles, birds, and 

 mammals, the nervous sys- 

 tem is composed of two prin- 

 cipal masses, the spinal mar- 

 row (Fig. 9, c), which runs 

 along the back, and the 

 brain, contained within the 

 skull.* The volume of the 

 brain is proportionally larger, 

 as the animal occupies a 

 more elevated rank in the 

 scale of being. Man, who 

 stands at the head of Crea- 

 tion, is in this respect also, the 

 9 most highly endowed being. 



62. The brain and spinal marrow give origin to the 

 nerves, which are distributed, in the form of branching 

 threads, through every part of the body. The branches 



* The brain is composed of several distinct parts which vary greatly, in 

 their relative proportions, in different animals, as will appear hereafter. 

 They are: 1. The medulla oblongata ; 2. Cerebellum; 3. Optic lobes; 

 4. Cerebral hemispheres ; 5. Olfactory lobes. See figures 9 and 21. 



