ACTION OF DRUGS ON THE VAGUS-HOOTS. 290 



which, like atropia, stimulates the vagus-roots, but, instead 

 of paralysing the ends, increases their sensibility, and thus 

 greatly augments the effect which the excited roots would have 

 exercised over the heart, even had the ends remained unaltered. 



Are the Vagus-roots Irritated directly hy the Drug or indirectlij 

 through increased Blood-pressure ? — Along witii the slow pulse,, 

 produced by the injection of atropia into the carotid, a rise occurs 

 in the blood-pressure ; and how are we to determine whether 

 the irritation of the vagus-roots is due to this increase, or to- 

 the direct action of the drug^itself ? This is a question very 

 difficult to solve in the case of atropia, on account of the- 

 rapidity with which the vagus-ends are paralysed and all influence 

 of the root over the heart destroyed. In the case of other drugs,, 

 however, where time is allowed, the question might be settled 

 by diminishing the blood-pressure and seeing whether or not 

 the slow pulse returned to its normal rate, and then raising it 

 again and observing whether the pulse again became slow. 



Mode of Lowering and Raising the Blood-pressure. — Tlie blood- 

 pressure may be lowered by opening a large artery, such as the 

 carotid or crural, and allowing the blood to flow out into a 

 vessel warmed to 98° Fahrenheit, and again raised l)y injecting 

 the warm blood back into the artery. Or we may adopt Ludwig 

 and Asp's plan, of inserting into the central end of the carotid 

 a straight tube with a stopcock in its middle, and the moist. 

 bladder of a small animal, well emptied of air, tied to its free 

 end. When the stopcock is opened, the blood rushes from 

 the carotid into the bladder, and the tension in the arteries is 

 diminished ; but, wlien we press the blood out of the bladder 

 back into the arteries, the tension on them is again increased. 



Are the Vagus-roots Irritated ReJleoAy from some other part of 

 the Nervous System .? — There are two ways of deciding this : 

 the first is to inject the poison in such a manner that it shall 

 reach the vagus-roots before it reaches the other nervous struc- 

 tures through which we suspect it to act reflexly ; the second is 

 to remove these nervous structures themselves, or to destroy 

 their function by means of some other poison. Thus, if we think 

 that atropia, when injected into the carotid, acts on the medulla 

 through the cerebrum, we may either remove the latter, or 



