114 PHYSIOLOGY. 



several little lobes, and if the two hemispheres be divided by 

 a knife, a beautiful appearance is presented, resembling the 

 trunk and branches of a tree ; composed of the white and 

 grey matter of the brain. Trom its shape it has been called 

 arbor vita, or tree of life. | 



12.V The Spinal marrow is a round cord of nervous matter 

 which passes down through the spine to the loins. It is di- 

 vided into two equal lateral parts, by a deep fissure or groove, 

 which runs through its whole length both on its front and 

 back surface./ Some say that it consists of four distinct 

 columns ; others that it is made up of six white strands, two 

 before, two behind, and two on the sides ; proceeding from 

 the cerebrum, the restiform bodies, and the cerebellum. 



13. (There are forty-three pair of nerves, making 86 in all, 

 proceeding from the brain and spinal marrowy These 

 nerves run to every part of the system, conveying, as already 

 stated, all sensations and impressions to the brain, and also 

 volition from the brain, to all the voluntary muscles of the 

 body. Of these, two pair go from the cerebrum, viz., those 

 for smell and sight, or the olfactory and optic ; five pair go 

 from the pons varolii, which is that round mass of nerve, 

 which lies between, and seems to connect the cerebrum and 

 cerebellum. Of these, the third, fourth, and sixth pair go to 

 the muscles of the eye ; while the ffth and seventh are dis- 

 tributed over the face. 



14. The remaining thirty-six pairs go out from the spinal 

 marrow, viz., Jive from the medulla oblongata ; eight from the 

 cervical ; twelve from the dorsal ; Jive from the lumbar ; and 

 six from the sacral portion of the spinal cord. These nerves 

 supply filaments to every muscular fibre in the system ; or 

 pass directly to the organs of sense without sending off twigs; 

 some of them are chiefly employed to form plexuses, or a net- 

 work of nervous threads, woven closely together, so that it 

 cannot be unravelled. 



