THE AUTONOMIC NERVOUS SYSTEM 879 



composed entirely of fibers belonging to voluntary effector neurons, but 

 also of connector fibers of the involuntary system. These are distin- 

 guished from the voluntary fibers by being much smaller in diameter; 

 indeed it was by this characteristic that Gaskell succeeded in tracing the 

 course of the involuntary fibers. 



In brief, therefore, the fibers of the internuncial neurons of the volun- 

 tary nervous system are confined within the central nervous system, 

 where they are contained mainly in the white columns of the spinal cord, the 

 pyramidal tracts, for example, being composed of internuncial fibers 

 from the cerebral neurons; the corresponding fibers of the involuntary 

 nervous system (connector), on the other hand, run out of the cord with 

 the anterior roots to effector neurons situated either in the ganglia of 

 the sympathetic chain or in peripheral localities. Just as the voluntary 

 internuncial fibers give off many collaterals, so do those of the involun- 

 tary system, so that an impulse transmitted by one internuncial neuron 

 may excite a broad field of effectors. We shall see later that it is through 

 these collaterals that reflex responses can apparently often be excited 

 by the stimulation of the central ends of nerves such as the hypogastric 

 to the bladder after all connections with the central nervous system 

 have been severed. (Fig. 230.) 



To elucidate the further course of the involuntary fibers, and deter- 

 mine the location of the effector neuron nerve cells, it becomes necessary 

 to supplement anatomical with physiological methods of investigation. The 

 various functions of the innervated parts vascular changes, muscular 

 movements, glandular activity are observed by the usual methods of 

 the physiologist, and the nerve roots or nerves believed to contain the 

 involuntary fibers either cut or stimulated. If a change is observed in 

 the functions, it indicates that part at least of the involuntary nerve 

 supply is contained in the nerve structure that has been cut or stimu- 

 lated. Such a result does not, however, inform us as to whether the 

 fiber is that of the connector or effector neuron whether it is pre- 

 ganglionic or postganglionic. This may be determined in many cases 

 by observing whether nerve degeneration occurs as a result of cutting 

 the fibers, but the most useful method for answering the question is 

 that discovered by Langley by the use of nicotine, which in certain con- 

 centrations specifically paralyzes the synaptic connections between the 

 connector and the effector neurons. If a weak (1 per cent) solution of 

 this alkaloid is painted on a ganglion or peripheral nerve plexus in 

 which the connector neuron finds its effector nerve-cell, it will break 

 the nerve path, so that physiological responses produced by stimulating 

 the preganglionic fibers become no longer elicitable. When the involun- 

 tary connector fibers run through several ganglia, as in the sympathetic 



