5 66 TEXT-BOOK OF PHYSIOLOGY. 



one cell station, between the spinal cord and the tissues. Though 

 innervated by the spinal cord, these structures receive their nerve 

 impulses, as previously stated, not directly but indirectly through 

 the ganglion cells. The meclullated nerve-fibers coming from the 

 spinal cord are known as pre- gang! ionic fibers; the non-medullated 

 fibers, passing from the ganglia, a,s'post-ganglionic fibers. 



The gray rami are composed of non-medullated nerve-fibers, 

 axons of the cells in the vertebral or lateral ganglia. After their 

 emergence from the ganglia they take a backward direction and 

 enter the spinal nerve-trunks, in company with which they pass to 

 the periphery, to be finally distributed to structures in the skin: viz., 

 non-striated muscles of blood-vessels, non-striated muscles of the 

 hair-follicles and epithelium of glands. They may therefore be 

 regarded as having vaso-motor, pilo-motor, and secretor functions. 



Afferent Sympathetic Fibers. With the foregoing groups of 

 efferent fibers, the sympathetic nerves, in the thoracic and lumbar 

 regions more especially, contain a number of afferent fibers which 

 when stimulated give rise to sensations of pain or to reflex phe- 

 nomena. The routes by which these afferent fibers reach the spinal 

 cord lead, on the one hand, into and through the gray rami to the 

 ganglia on the posterior roots, where they have their cells of origin; 

 and, on the other hand, into and through the white rami. The 

 number of afferent fibers in any trunk in comparison with the effer- 

 ent is quite small. 



FUNCTIONS OF THE SYMPATHETIC SYSTEM. 



view according to which the sympathetic system is to be 

 regarded as an independent apparatus endowed with functions of its 

 own and in nowise directly dependent for its activities on the spinal 

 cord, is at the present time discarded. Peripheral structures cease 

 to exhibit their characteristic^ Junctions after division of the spinal 

 nerves in connection with their related ganglia. This does not 

 exclude the possibility of the sympathetic cell-body, in virtue of the 

 interchanges between it and the blood and lymph by which it is 

 surrounded, maintaining its own nutrition and exerting a favor- 

 able influence over the nutrition of the peripheral tissues to which 

 its efferent branches are distributed. 



The nerve-tissue in its entirety may be regarded as a single 

 system which may be functionally divided into a nerve system of 

 animal and a nerve system of vegetative life, according as the nerve 

 energies originating in and emanating from the central nervous 

 system are transmitted directly to the skeletal muscles or indirectly, 

 through the intervention of a sympathetic neuron, to visceral muscles 

 and glands. In the former system but one neuron, the spino-periph- 



