THE SYMPATHETIC SYSTEM 309 



traction towards C, or in decreasing the traction towards B. 



Applying this notion to the pupillary reaction, we 

 know that: 



(a) The parasympathetic innervates the sphincter of the 

 iris and that stimulation of it will cause myosis; 



(6) The sympathetic innervates the irido-dilator and its 

 stimulation causes mydriasis. If, therefore, we pharma- 

 cologically wish to produce mydriasis, we act in the 

 following manner: 



(a) By paralyzing the parasympathetic (atropin). 



(6) By stimulating the sympathetic (adrenalin). 



FIG. 31. Showing diagrammatically the mechanism of disturbance of equilibrium. 



We cannot insist too much on these facts, for their 

 importance is tremendous. This principle is the basis for 

 the greater portion of our knowledge of the physiology and 

 pathology of the neuro glandular systems of vegetative 

 life. It is through these principles that we are able to 

 understand the action of reciprocity of the nervous and 

 endocrine elements. 



IV. TOTAL TONTJS. PARTIAL TONUS. 



We have seen that there exists, under the influence of 

 certain substances, a state of tonus of all or part of the 

 nervous systems of vegetative life. This tonus may 

 manifest itself as a stimulation or depression. Finally, in 



