TO WHAT CAUSE IS A QUICK PULSE DUE? 205 



Is the Quicheniiig due to Farali/sis of the Vagi .^^The exclu- 

 sion of the other causes leads us to believe that it is due to this ; 

 but, in order to avoid the possibility of error, we must try to 

 confirm our conclusion by other experiments ; and, moreover, we 

 liave still to find out which part of the vagus is affected — its 

 roots, its fibres, or its ends in tlie heart. 



Ai^e the Vagus-roots Paralysed hy the Drug ? — We are enabled 

 to answer this question by our knowledge of the fact, that 

 poisons only act on the parts to which they are carried by the 

 blood, and that wlien introduced into tlie circulation they do not 

 reach every part of the body at once, but are carried on with 

 the blood-stream first to one part and tlien to another, and will 

 reach a part near the point where they were introduced before 

 one which is farther off. Thus, if we inject a drug into the 

 carotid it will be carried direct to tlie head, and will act on the 

 medulla oblongata and the roots of the vagus before it reaches 

 the heart ; but if we inject it into the jugular vein it will reach 

 the heart and act on the vagus-ends in it before it reaches the 

 roots in the medulla. If atropia paralyse the vagus-roots, then 

 its injection into the carotid towards the head sliould be fol- 

 lowed by rapidity of the pulse more quickly than its injection 

 into the jugular ; but if it act on the vagus-ends in the heart, 

 the pulse should become rapid more quickly after injection into 

 the jugular vein towards the heart than after injection into the 

 carotid. On testing this experimentally, it is found that, when 

 atropia is injected into the jugular vein towards the heart, the 

 pulse at once becomes quick, even before the injection is 

 finished ; but, when it is injected into the carotid towards the 

 head, the pulse is not quickened for a quarter of a minute or 

 more, or, in other words, till the poison has had time to pass 

 through the capillaries of the head and go through the veins to 

 the heart. This, then, shows that it is the vagus-ends in the 

 heart, and not the roots in the medulla, that are paralysed 

 by it. 



Are the Vagus-fihres Paralysed'? — From the rapidity with 

 which paralysis of the vagus occurs after atropia reaches its 

 ends, we have already come to the conclusion that the ends are 

 the part affected rather than the roots or fibres ; but it is well to 



