•300 EXrEAiLMENTAL INVESTIGATION OF THE ACTION OF MEDICINES. 



.'ibolish its function by opium or chlovaL The application of 

 'irritating vapours, such as ammonia or tobacco-smoke, to the 

 fiasal mucous membrane of a rabbit, produces still-stand of the 

 heart. We ascertain that this is due to irritation of the vagus 

 by cutting it and finding that the vapour then has no effect ; 

 «,nd we next decide that the irritation is conducted to the 

 nervous centres through the tri(]jeminus and not through the 

 •olfactory nerve, by observing that section of the former like- 

 wise prevents the action of the vapour on the heart, while 

 ^section of the latter does not affect it. 



Aox the Vagus-7'oots Irritated indirectly hy the Drug imjpairivg 

 .Respiration, and thus alloiuing Carhonic Acid to accuimdaie in the 

 Blood 'I — In the experiment just mentioned, we have ascertained 

 that the vagus is irritated, and that irritation is conducted to 

 ■'the nerve-centres through the trigeminus, but we do not know 

 that the irritation is directly reflected from the trigeminus to 

 the vagus. It might be due to irritation of the vagus-roots by 

 <jarbonic acid, which has accumulated in the blood from im- 

 peded respiration ; for the irritating vapour applied to its nose 

 causes the rabbit to close its nostrils and stop breathing for a 

 •while if the trigeminus be intact, but when it is cut no irritating 

 impression can be conveyed to the brain, and so no closure of 

 the nostrils takes place, either voluutarily or reflexly. The 

 •rabbit, therefore, continues to breathe freely : no carbonic acid 

 •accumulates in the blood, and no irritation of the vagus occurs. 

 Other drugs, such as strychnia and curare, &c., impede respira- 

 tion — not by causing closure of the nostrils and consequent 

 'Obstruction to the passage of air to the lungs, but by acting on 

 the muscles and nerves and diminishing the respiratory move- 

 ments. Strychnia does this by jDroducing tetanic contraction of 

 the respiratory muscles, curare by paralysing them, and chloral 

 by diminishing the excitability of the respiratory nervous 

 •centre. In all such cases, in order to ascertain that indirect 

 irritation of the vagus from impeded respiration is not the cause 

 •of the slowing of the pulse, we insert a cannula into the trachea 

 -and begin artificial respiration ; we then note the rate of the 

 pulse and blow the irritating vapour into the nostrils, or inject 

 the drug into the veins, and see whether or not the pulse is 



