194 THE HUMAN BODY 



Because of their positions with regard to sympathetic ganglia the 

 first and second neurons are known respectively as pre-ganglionic 

 and post-ganglionic neurons. The latter present the anatomical 

 peculiarity of being for the most part devoid of myelin sheaths; 

 nerve-trunks made up of post-ganglionic fibers can therefore be 

 distinguished from other nerve-trunks by their gray color. 



The structures innervated by the autonomic system perform 

 their functions by acting to a considerable extent in groups to- 

 gether; not individually as do skeletal muscles. To enable them 

 to be stimulated in groups single autonomic pathways commonly 

 involve numerous end structures. This is accomplished 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 Effect of Nicotine. Much of our knowledge of the auto- 

 nomic 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 autonomic innervation of the 

 eye. The size of the pupil is regulated by opposing autonomic 

 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 Autonomic System. The autonomic 

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

 ing pathway by which influences are brought to bear on the struc- 

 tures 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 

 smooth muscles and glands. The autonomic system is, therefore, 



