NERVOUS SYSTEM AND GENERAL SENSATION. 



45 



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

 ume of the nervous system 



va.ry 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 svs- 



j 



tern 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 

 most highly endowed being. Fig. 9. 



62. With the brain and spinal marrow are connected the 

 nerves, which are distributed, in the form of branching 

 threads, through every part of the body. The branches 

 which unite with the brain are twelve pairs, called the cere- 



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

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

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

 4. Cerebral hemispheres ; 5. Olfactory lobes ; 6. the pituitary body ; 7- 

 the pineal body. (See figures 9 and 21.) The spinal marrow is made up 

 by the union of four nervous columns. 



