CHAP. XVIII.] THE NERVOUS SYSTEM. 



215 



spinal cord, and the fibres 'that arise from them are mostly of 

 the non-medullated variety. 



These ganglia and nerves do not form an independent ner- 

 vous system, for each ganglion is connected by motor and sensi- 

 tive fibres with the cerebral system. The sympathetic nerves are 

 distributed to the viscera and blood-vessels, of which the move- 

 ments are involuntary, and the general sensibility obtuse. They 

 form networks or plexuses upon the heart, about the stomach, 

 and other viscera in the trunk ; they also enter the cranium, 

 send branches to the organs of special sense, and, in particu- 

 lar, influence the pupil of the eye. Their most important 

 distribution, however, is in connection with the blood-vessels. 

 They form plexuses around the vessels, 

 especially the arteries, and send fibres 

 to terminate in the involuntary mus- 

 cular tissue of which the walls of 

 these tubes are largely composed. The 

 nerves thus distributed are called 

 " vaso-motor " nerves. 



The unity of the nervous system. 

 From the foregoing description of the 

 nervous system, it is clear that it forms 

 one continuous whole, through the 

 agency of which all the varied activi- FlG - 125 - NERVE ENDING 



,, . . , .. IN MUSCULAR FIBRE OF A 



ties ol the body are controlled and LIZARD. (Kuhne.) The end- 

 regulated. We may compare it to a plate, or motorial ending of the 



' axis-cylinder, is seen sideways. 



telegraphic system, the central office 



of which would represent the brain and spinal cord, the more 

 important sub-offices the sympathetic ganglia, and the minor 

 offices the isolated ganglia ; while the telegraph wires, directly 

 or indirectly uniting all, would correspond to the nerves. And 

 just as a message started along some outlying wire may be 

 transmitted to a central office, and, according to the informa- 

 tion received, will be stopped there, or sent to some other 

 centres to be forwarded by them in one direction, in two direc- 

 tions, or all over the country, so may a nervous impulse reach- 

 ing the brain merely excite some change in the brain itself, or 

 be sent out from thence along the nerve-trunks to excite activi- 

 ties in various parts of the body. 



Peripheral termination of nerves. The nerves have their origin 



