THE SYMPATHETIC NERVOUS SYSTEM 227 



sympathetic system includes those neurons which put the periph- 

 eral autonomous system into functional connection with the 

 central nervous system, thus providing a central regulatory con- 

 trol over the autonomous system. This part of the sympa- 

 thetic nervous system includes the peripheral courses of the 

 neurons involved in the general cerebro-spinal visceral reflex sys- 

 tems (see pp. 76, 89, 93). 



The peripheral autonomous nervous system appears to be a 

 direct survival of that diffuse type of nervous system which is 

 found in the lowest animals which possess nerves at all, such as 

 some jelly-fishes and worms. The central nervous system of 

 higher animals is supposed to have developed by a concentration 

 of ganglia in such a diffuse system (see p. 27), a portion of which 

 remains as the peripheral autonomous sympathetic system (Fig. 

 17, p. 53). But during evolution the central nervous system 

 increased in importance for integrating and regulating the 

 functions of the body, the central control of the viscera assumed 

 greater importance, and the general cerebro-spinal visceral sys- 

 tems were developed to serve this function. 



Figure 56 (p. 126) illustrates the typical arrangement of the 

 visceral sensory and motor fibers in the spinal nerves, and their 

 relations to the sympathetic ganglia and nerves. These fibers, 

 of course, belong to the cerebro-spinal visceral systems; the 

 peripheral autonomous system is not included in the diagram. 

 The central control of the visceral apparatus is effected (1) by 

 afferent visceral nerve-fibers distributed peripherally through 

 the sympathetic nerves and entering the spinal cord through the 

 dorsal spinal roots, and (2) by efferent visceral nerves which 

 leave the spinal cord through the ventral roots and also enter the 

 sympathetic nerves. In lower vertebrates (and possibly also in 

 man) some of these fibers leave by the dorsal roots also. 



The cell bodies of the afferent neurons lie in part in the 

 spinal ganglia and in part in the sympathetic ganglia. Figure 

 109 illustrates the connections of these two types of afferent 

 visceral neurons. Neuron 3 of this figure may transmit its 

 impulse either directly into the spinal cord through its centrally 

 directed process or by a collateral branch to some other cell 

 body of the spinal ganglion (neuron 1). The fiber marked 4 

 arises from a cell-body lying in some sympathetic ganglion and 



