166 THE HUMAN BODY 



by rich branching of the pre-ganglionic fibers, enabling each to 

 have synaptic connection with a number of post-ganglionic 

 neurons, and so to influence simultaneously numerous end organs. 



The heart and most of the abdominal organs receive part of 

 their innervation by way of the tenth cranial nerves, the vagi. 

 Although here the pathway is not by the sympathetic system 

 proper, it is constructed on the true sympathetic plan; there 

 being in each case two neurons, pre-ganglionic and post-ganglionic. 

 Connection between the two occurs in ganglia placed conveniently 

 to the organs affected. Those of the heart, for example, are im- 

 bedded within its mass, and are known as the cardiac ganglia. 

 The sympathetic system of the head is also in large part not 

 connected with the sympathetic system proper; here again, how- 

 ever, the true sympathetic structure obtains. 



The Effect of Nicotine. Much of our knowledge of the sym- 

 pathetic system has resulted from the discovery that application 

 of the drug nicotine to sympathetic ganglia prevents the passage 

 of impulses over whatever synapses may be contained therein. 

 By the use of this drug, therefore, the point of contact of pre- 

 ganglionic with post-ganglionic fibers in the pathway to any 

 particular organ can be determined. To illustrate how its use 

 brings out these points of contact we may take the sympathetic 

 innervation of the eye. The size of the pupil is regulated by 

 opposing sympathetic fibers; one set tending to constrict it, the 

 other to dilate it. By the use of nicotine it has been shown that 

 the contact of pre-ganglionic with post-ganglionic fibers in the 

 constrictor pathway is in the ciliary ganglion, which is in the 

 orbit, while for the dilator pathway the connection between pre- 

 ganglionic and post-ganglionic fibers is in one of the sympathetic 

 ganglia of the neck. 



Reflex Control of the Sympathetic System. The sympathetic 

 system, as we have seen, forms only the last step in the conduct- 

 ing pathway by which influences are brought to bear on the 

 structures it innervates. Like the motor system for the skeletal 

 muscles it conveys only those impulses which are imparted to it 

 from without. It is, in other words, the efferent portion of a 

 reflex mechanism. 



The so-called "vital" processes of the Body are, with the ex- 

 ception of respiration, largely carried on through the agency of 



