CHANGES IN THE CIRCULATORY SYSTEM. 467 



ment and high arterial pressure, and muscular rest by calm 

 action of the heart and a quiet pulse. 



II. PHAKMACODYNAMICS. 



The circulatory system affords one of the most striking 

 instances in the hody of provisions for physiological change, and 

 of functional reaction to influences of every kind which bear, or 

 may be brought to bear, upon it. Herein lie at once its power 

 of accommodation to circumstances and its vulnerability ; and 

 here, too, the therapeutist discovers his opportunity of in- 

 fluencing the heart and vessels at his pleasure. 



1. The total volume of blood in circulation being one of 

 the prime factors of the blood pressure, every change in this 

 volume, whether by abstraction or addition, must alter the 

 pressure. This can readily be accomplished by leeching, cup- 

 ping or venesection on the one hand, or by transfusion on the 

 other hand. As a matter of fact, however, the effect of either 

 method on the circulation is but temporary. The tension of the 

 pulse falls with venesection, only to rise again quickly by in- 

 creased absorption of fluids from the tissue and bowels into the 

 circulation. Transfusion raises the blood pressure for a time, but 

 the compensating mechanisms soon restore the previous average 

 pressure. Venesection is therefore the most powerful of all 

 measures for quickly taking the tension off the whole circulation, 

 and relieving the heart and lungs, but it is practically useless 

 for the purpose of permanently reducing the blood pressure ; 

 and transfusion is similarly of inestimable value in rapidly re- 

 storing the pressure, if it have fallen dangerously low from loss 

 of blood, and thus preventing death by circulatory failure. 



2. The Heart. a. The intrinsic nervo-muscular apparatus may 

 be either stimulated or depressed. The first direct cardiac 

 stimulant is an active coronary circulation, through which the 

 heart responds to improved quality of the blood in oxygen and 

 plasma, and thus, indirectly, to proper air and food, healthy 

 digestion, and hepatic action. Direct cardiac stimulants in- 

 clude many drugs, such as Alcohol, Digitalis, Scilla, Strychnia, 

 Ammonia, Ether, etc. The continuous battery current applied 

 through the region of the heart acts similarly. Reflex stimu- 

 lation is a ready and powerful means of increasing the activity 

 of the heart, or of rousing it in actual arrest, and includes the 

 various methods of local nervous stimulation described in 

 chapter xi. , especially irritation of the fifth nerve by Ammo- 

 nia, the cold douche and flagellation, and counter-irritation 

 of the praecordium. Cupping and leeching also exert a stimu- 

 lant influence on the heart through the nervous system, as well 

 as relieving it by abstraction of blood. Carminatives stimulate 



