THE SYMPATHETIC AND PARASYMPATHETIC SYSTEMS 635 



confined to the pilomotor impulses, but also involves other sympathetic 

 impulses. 



Afferent Conduction in the Autonomic System. — We have noted 

 that those fibers of the anterior horn which eventually enter the white 

 ramus communicans, terminate around the cells of the first sympa- 

 thetic ganglion (Fig. 314, S). The axons of the latter either return 

 to the spinal nerve by way of the gray ramus communicans or continue 

 within this system to other more distant ganglia. The neuron form- 

 ing the connection between the cord and the sympathetic ganglion 

 constitutes the preganglionic path (P), and the one situated on the 

 distal side of the ganglion, the postganglionic path (Po). This termin- 

 ology, however, is not always indicative of real conditions, because 

 some of the preganglionic fibers rnay pass directly through the first 

 sympathetic ganglia without entering into communication by synapse 

 with these cells. According to Langley, the precise nature of a 

 certain sympathetic fiber may be ascertained by moistening the 

 gangUon with a solution of nicotin (N), This agent, it will be remem- 



"^ 4 



i 



Fig. 314. — Diagram to Illustrate the Action of Nicotin. 

 C, Spinal cord; P, preganglionic path; S, sympathetic ganglion; Po, postganglionic 

 path; E, effector; /, neuron which does not form a synapse in »S; II, neuron forming 

 synapse in S; N, destroys connections in synapse, blocking nerve impulse in neuron 

 // but not in /. 



bered, first stimulates and then paralyzes the cells, preeminently at 

 their junction with the axon terminations of the central neurons. 

 Consequently, the stimulation of the preganglionic path must remain 

 without effect if the fibers composing the latter actually enter into 

 synapses within the nicotinized area (II). The reason for this is 

 that the nicotin has produced a block within the ganglion. Conversely, 

 if the central fibers traverse the ganglion without entering into com- 

 munication with other cells (I), they must necessarily retain their 

 power of conducting impulses to peripheral parts, because the nicotin 

 does not affect the nerve-fibers. In the latter case, therefore, the 

 excitation of the preganglionic path must give rise to motor effects. 

 It is true, however, that this method does not allow of a universal 

 application, because certain animals, such as the dog, are very re- 

 sistant against this agent; in fact, its action differs even in the same 

 animal when applied to different structures. Thus, it has been found 

 that the cervical ganglia are much more susceptible to it than the 

 ganglia of the splanchnic area. 



The autonomic nervous system is essentially a distributing mechan- 

 ism and hence, its ganglia may be said to serve primarily the purpose 



