DRUGS ACTING ON THE CIRCULATION 37 



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 systemicaUy to contract vessels. 



(b) Drugs acting systemicaUy to dilate vessels. 



Amyl nitrite Chloral 



Nitroglycerin Aconite 



Spirit of nitrous ether Opium 



Alcohol Belladonna (secondary action) 



Ether Hyoscyamus" " 



Chloroform Stramonium " 



(c) Agents acting locally to contract vessels. 

 Cold ' Astringents 



(d) Agents acting locally to dilate vessels. 

 Heat Counter-irritants 



tJses. — Drugs or agencies causing general dilatation of 

 vessels are useful in overcoming internal congestions and 

 colds by equalizing the circulation ; that is, by causing the 

 blood to be distributed more equably about the body. They 

 are also of benefit in morbid conditions attended with a 

 high, vascular tension; and are serviceable in dilating 

 peripheral vessels and in causing perspiration and loss of 

 heat in fevers (spirit of nitrous ether and alcohol). Drugs 

 inducing general contraction of vessels are employed in 



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