274 EXrERIMENTAL INVESTIGATION OF THE ACTION OF INIEDICINES. 



time to fill completely, and so sends out little blood at each' 

 beat; but, when going slowly, it becomes quite full during each 

 diastole, and sends out a larger c^uantity of blood at each 

 contraction. 



It must be remembered that we measure the blood-pressure - 

 in the systemic arteries ; and, before the blood can get into 

 them from the veins, it must come through the pulmonary 

 vessels. Any contraction of the lumen of these vessels, by 

 lessening the entrance of blood into the systemic arteries, will 

 cause the pressure' in them to fall. * 



Influence of Nerves on Blood-Pressuee. — Both the quick- 

 ness of the heart's beat and the contraction of the arteries are 

 regulated by the nervous system ; and it is generally by acting 

 on different parts of it that drugs alter the blood pressure, 

 though they may also do so by acting on the muscular walls of 

 the heart and arteries themselves. The parts of the nervous 

 system chiefly concerned in regulating the circulation are : 



I. The cardiac ganjlia which lie in the w^alls of the heart, 

 and are, in all probability, the cause of its rhythmical action. 



II. Inhihitory nerves, which render the heart's action slow, 

 and, if irritated very strongly, may stop its beating altogether, 

 and produce still-stand in diastole. The inhibitory fibres have 

 their origin or roots in the medulla, and proceed in the vagi to 

 the heart. In man and in dogs, they are normally in constant 

 iii,ction ; and, after they are cut or paralysed, the heart beats in 

 the dog three or four times as quickly, and in man twice as 

 quickly, as before. In rabbits and cats they act less, and their 

 division only makes the heart go one-half or one-fourth 

 faster. A drug may irritate them, and render the heart's action 

 slow — 



1. By acting directly on (a) their roots in the medulla, 

 {h) their fibres, (c) their ends in the heart ; 



2. Indirectly, through its action on other parts, producing 

 (a) increased blood-pressure, or (b) accumulation of carbonic 

 acid in the blood, both of which act as irritants to the vagus- 

 roots ; 



3. Reflexly, (a) through irritation of sensory nerves, (b) irrita- 

 tion of the intestines, (c) of the sympathetic nerve, {d) of the 



