672 TEXT BOOK OF PHYSIOLOGY 



near the group of nerve-cells which gives origin to the nerve-fibers for the sphincter 

 muscle of the iris. It is also believed that the accelerator nerves for the heart 

 which emerge from the spinal cord in the second and third thoracic nerves come 

 from a center lying in the bulbar region not far from the center from which the 

 inhibitor fibers in the vagus come. 



Nevertheless there are other instances in which this antagonism does exist, e.g., 

 in 'the effects of the efferent fibers of the vagus and the splanchnic nerves on the 

 muscle- wall of the intestine, the former augmenting, the latter inhibiting the con- 

 traction and so in other regions. But this does not necessitate the employment of 

 new terms. It suffices to say that there is a difference in the action of different 

 parts of the general autonomic system. It is just as probable, however, that the 

 difference in the effects observed following stimulation of the two classes of nerves 

 depends on the character of their peripheral terminations rather than on a differ- 

 ence in the character of the central cells. 



Huber, in a recent review of the morphology of the sympathetic system, says 

 in regard to the foregoing division, "such a division of the autonomic nerve system 

 does not seem justifiable when viewed in the light of a morphologic study of the 

 ganglia with their constituent neurons, the terminations of the neuraxis of the 

 neurones in the various tissues, and the connections of these neurones with the 

 cerebrospinal axis by means of the pre-ganglionic fibers; morphologically con- 

 sidered, the entire autonomic system is a unit and will be treated as such. The 

 minor structural differences, more apparent than real, observed in the neurons of 

 certain of the cranial autonomic ganglia and in the entire system, do not warrant, 

 it would seem to me, a regional subdivision of the autonomic system, when con- 

 sidered from the viewpoint of structure." 



