36 GENERAL ACTIONS OF DRUGS 



ammonia, ammonium carbonate, camphor, caffeine, strych- 

 nine, atropine, aconite and veratrum viride. The reader is 

 referred to special articles on these drugs for therapeutical 

 indications and other details. 



III. — Drugs Acting on the Blood Vessels. 



The following table includes the mechanism regulating 



vascular tension : 



Smooth muscular fibres 

 1. In the walls of the vessels. ^ Terminations of vasodilators and vaso- 

 constrictors 



I 



Nervesupplyof .eseeU.... \ ^t^i'„t,7,tors 



Vasomotor centres in the medulla and 

 subsidiary centres in the spinal 

 3. Centres •{ cord and sympathetic system, con- 

 trolling the vasodilating and con- 

 stricting nerves 



Each vessel is governed by two sets of fibres, — the 

 constricting and dilating, — but we cannot discriminate 

 between the action of a drug on the muscular fibres and the 

 peripheral nerve endings in the vessel walls ; nor can we 

 always tell whether a drug acts to stimulate one set of 

 peripheral fibres or depress the other. 



Vascular tension is increased not only by contraction of 

 vessels, but also by drugs which cause the heart to beat more 

 quickly, and by those making its pulsations more forcible 

 and complete, so that all the blood is Squeezed out of the 

 ventricle at each contraction. Contrariwise, blood pressure 

 is diminished, not only by those drugs inducing vascular 

 dilatation, but by those reducing the rate or force of the 

 heart, or both. 



We shall simply classify drugs influencing the vessels 

 according as to whether they act after absorption into the 

 blood, or only when applied locally to the vessel walls. 



(a) Drugs acting systemically to contract vessels. 



Ergot Squill 



Digitalis Sparteine 



Strophanthus Strychnine 



