21t) THE MICROSCOPIST. 



ence between this and the cerebro-spinal system is that in 

 the latter the nerve-cells form large masses, and the union 

 of its parts is effected by means of central fibres, while in 

 the sympathetic the cells are more widely separated, and 

 union between them and with the cerebro-spinal axis is 

 by means of peripheral fibres. The sympathetic is con- 

 sidered a motor and sensitive nerve to internal viscera, 

 and to govern the actions of bloodvessels and glands. 



2. The cerebro-spinal system, divided into : 



(1.) A system of ganglia subservient to reflex actions, 

 the most important of which is the spinal cord, where the 

 gray or vesicular nervous matter forms a continuous tract 

 internally. 



(2.) A gangliouic centre for respiration, mastication, 

 deglutition, etc., with a series of ganglia in connection 

 with the organs of special sense: the medulla oblorigata, 

 with its neighboring structures; the mesocephalon, cor- 

 pora striata, and optic thalami. 



(3.) The cerebellum, a sort of offshoot from the upper 

 extremity of the medulla, for adjusting and combining 

 voluntary motions. 



(4.) The cerebrum, cerebral hemispheres, or ganglia, 

 which are regarded as the principal organs of voluntary 

 movements. In the lower vertebrates the hemispheres 

 are comparatively small, so as not to overlap the other 

 divisions of the brain ; but in the higher Mammalia they 

 extend over the olfactory lobes and backward over the 

 optic lobes and cerebellum, so as to cover these parts, 

 while they also extend downward toward the base of the 

 brain. In the lower vertebrates, also, the surface of the 

 hemispheres is smooth, while in the higher it is compli- 

 cated by ridges and furrows. 



(5.) The cerebral and spinal nerves. The spinal nerves 

 arise in pairs, generally corresponding with the vertebrae. 

 Each has two roots, one from the dorsal, and one from 

 the ventral region of its half of the cord. The former 



