298 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



to find out Avhat effect the drug has on the vagus-roots them- 

 selves, Ave must inject it into the carotid, so that it may reach 

 them Ijefore it reaches the fibres or ends ; and note wliat change 

 occurs in the pulsations of the heart immediately after the 

 injection. Any change which occurs immediately is due to the 

 effect of the drug on tlie roots themselves ; and, by comparing 

 the number of pulsations at this time with that wliich is found 

 a quarter of a minute or so afterwards, when the drug has- 

 reached the vagus-ends, we may discover whether their excita- 

 bility has been increased or diminished. Thus, in the experi- 

 ment already mentioned for ascertaining whether or not the . 

 vagus roots are paralysed by atropia, we find that, when we 

 inject it into the carotid, we get immediate slowness of the 

 pulse, showing that the vagus-roots are irritated by the drug ;, 

 but whenever it gets round to the heart it paralyses the vagus- 

 ends, and the slowness at, once disappears. If we were to keep 

 the head alive by supplying it with an artificial stream of blood 

 containing atropia, and prevent any of the poisoned blood irom 

 reaching the heart, the slowness might be continued in- 

 definitely. 



Mode of svjjqulyhuj the Head and Body tvith di fere at kinds 

 of Mood. — In his researches on respiration Heriug employed 

 a method of this sort, at one time supplying the brain with 

 blood loaded with carbonic acid while the blood of the ])ody was- 

 richly arterialised, and at another sending arterial blood to the 

 brain while respiration was stopped and the l)lood circulating 

 in the body was intensely venous. For this purpose he opened 

 the thorax and tied the left carotid and innominate close to the 

 aorta, and the vena cava superior close to the heart in a cat ; he 

 then introduced one cannula into the innominate artery, and 

 another into the vena cava, and injected dog's blood, defi- 

 brinated and warmed, into the innominate srtery, while he 

 allowed it to flow out by the vein. 



In atropia we have an example of a drug which acts on more 

 than one part at once, and whose action on one part completely 

 neutralises the effect wliich its action on the other would pro- 

 duce. In the case of others, however, we have the action on 

 the ditl'erent parts strengthening each other, as in veratria 



