THE SYMPATHETIC NERVOUS SYSTEM. 277 



semicircular canals. It is believed by some that the constant 

 movements of the fluid in the semicircular canals is the cause 

 of sea sickness. The unusual nature of these movements causes 

 confusion in the impressions transmitted to the cerebellum from 

 the canals, but after a while the cerebellum becomes accustomed 

 to them and the sea sickness passes away. 



The Sympathetic Nervous System. 



Along with the vagus and one or two less prominent cere- 

 brospinal nerves, the sympathetic constitutes the autonomic 

 nervous system, so called because it has to do with the innerva- 

 tion of automatically acting structures, such as the viscera, the 

 glands and the blood vessels. The characteristic structural fea- 

 ture of the nerves of this system is that they are connected 

 with nerve ganglia located outside the central nervous system. 

 In these ganglia the nerve fibers run to nerve cells, around which 

 they form synapses, thus permitting the nerve impulse to pass 

 on to the cell, which then transmits it to its destination along its 

 own axon (see p. 241). Before arriving at the ganglion in 

 which the synapsis is formed, the 'fibers are called pregan- 

 glionic; after they leave, they are called postganglionic. A 

 preganglionic fiber .may run through several ganglia before it 

 becomes changed to a postganglionic fiber. In the case of the 

 vagus and other cerebral autonomic nerves, the ganglia are 

 often situated, as in the heart (see p. 185), at the end of the 

 nerve, but in the case of the sympathetic itself, they are more 

 numerous, and are mainly situated at the sides of the vertebral 

 column, where, along with the connecting fibers, they form a 

 chain the sympathetic chain which can easily be seen on 

 opening the thorax and displacing the heart and lungs. 



Two fine branches connect each of the spinal nerves with 

 the corresponding sympathetic ganglion. It is through one of 

 these branches that the sympathetic chain receives its fibers 

 from the spinal cord. Through the other, fibers run from the 

 ganglion to the spinal nerve. Some of the sympathetic ganglia 

 are situated at a distance from the spinal cord ; the ganglia 

 which compose the solar and hypogastric plexuses are examples. 



