CHAPTER XVIII. 



THE NERVOUS SYSTEM. 



Chief Divisions of the Nervous System. The Nervous System 

 consists of two portions or systems, the (I.) Cerebro-spinal, and the (II.) 

 Sympathetic. 



(I.) The Cerebro-spinal system includes the Brain and Spinal cord, 

 with the nerves proceeding from them. Its fibres are chiefly, but not 

 exclusively, distributed to the skin and other organs of the senses, and to- 

 the voluntary muscles. 



(II.) The Sympathetic Nervous system consists of: (1) A double chain 

 of ganglia and fibres, which extends from the cranium to the pelvis, along 

 each side of the vertebral column, and from which branches are distributed 

 both to the cerebro-spinal system and to other parts of the sympathetic 

 system. With these may be included the small ganglia in connection with 

 those branches of the fifth cerebral nerve which are . distributed in the 

 neighborhood of the organs of special sense: namely, the ophthalmic, otic, 

 spheno-palatine, and sulmaxillary ganglia. (2) Various ganglia and 

 plexuses of nerve-fibres which give off branches to the thoracic and ab- 

 dominal viscera, the chief of such plexuses being the Cardiac, Solar, and 

 Hypogastric; but in intimate connection with these are many secondary 

 plexuses, as the aortic, spermatic, and renal. To these plexuses, fibres 

 pass from the prgevertebral chain of ganglia, as well as from cerebro-spinal 

 nerves. (3) Various ganglia and plexuses in the substance of many of 

 the viscera, as in the stomach, intestines, and urinary bladder. These, 

 which are, for the most part, microscopic, also freely communicate with 

 other parts of the sympathetic system, as well as, to some extent, with the 

 cerebro-spinal. (4) By many, the ganglia on the posterior roots of the 

 spinal nerves, on the glosso-pharyngeal and vagus, and on the sensory root 

 of the fifth cerebral nerve (Grasserian ganglion), are also included as sym- 

 pathetic-nerve structures. 



Elementary Structure. The organs both of the Cerebro-spinal and 

 Sympathetic nervous systems are composed of two structural elements- 

 -fibres and cells. The cells are collected in masses, and are always min- 

 gled, more or less, with fibres; such a collection of cellular and fibrous 

 nerve-structure being termed a nerve-centre. The fibres, besides entering 

 into the composition of nerve-centres, form by themselves the nerves, 



