THE NERVOUS SYSTEM 



379 



In vertebrates, the nervous 

 developed, more complex, and 

 more concentrated than in the 

 lower forms. In fact, there are 

 some parts, as the brain, to 

 which we find nothing homolo- 

 gous in the invertebrates ; and 

 while the actions of the latter 

 are mainly, if not wholly, auto- 

 matic, those of backboned ani- 

 mals are largely voluntary. Its 

 position, moreover, is peculiar, 

 the great mass of the nervous 

 matter being accumulated on 

 the dorsal side, and inclosed 

 by the neural arches of the 

 skeleton. 



The brain and spinal cord 

 lie in the cavity of the skull 

 and spinal column, wrapped in 

 three membranes. Each con- 

 sists of gray and white nervous 

 matter; but in the brain the 

 gray is on the outside, and the 

 white within ; while the white 

 of the spinal cord is external, 

 and the gray internal. Both 

 are double, a deep fissure run- 

 ning from the forehead back- 

 ward, dividing the brain into 

 two hemispheres, and the spinal 

 cord resembling two columns 

 welded together ; even the 

 nerves come forth in pairs to 

 the right and left. The brain is 



system is more highly 



FIG. 334. Human Brain and Spinal 

 Cord, seen from below, about one- 

 tenth natural size ; , right hemi- 

 sphere of cerebrum; b, anterior 

 lobe; c, middle lobe; d, medulla 

 oblongata ; e, cerebellum ; f, first 

 spinal nerve ; g, brachial plexus 

 .of nerves supplying the arms ; 

 h, dorsal nerves ; i, lumbar nerves ; 

 k, sacral plexus of nerves for the 

 limbs ; /, cauda equina: the figures 

 indicate the twelve pairs of cranial 

 nerves, of which i is olfactory, 

 2 optic, and 8 auditory. 



