166 KESPIKATICXNT. 



settling some of these questions, which might be regarded as somewhat uncertain, we 

 instituted, a few years ago, a series of experiments upon the situation and cause of the 

 respiratory sense. In these observations, the following facts, some of which had been 

 previously noted, were demonstrated : 



1. If the chest be opened in a living animal, and artificial respiration be carefully 

 performed, inflating the lungs sufficiently but cautiously and taking care to change the 

 air in the bellows every few moments, as long as this is continued, the animal will make 

 no respiratory effort; showing that, for the time, the respiratory sense is abolished. 



2. When the artificial respiration is interrupted, the respiratory muscles are thrown 

 into contraction, and the animal makes regular, and at last violent efforts. If we now 

 expose an artery and note the color of the blood as it flows, it will be observed that the 

 respiratory efforts commence only when the blood in the vessel begins to be dark. "When 

 artificial respiration is resumed, the respiratory efforts cease only when the blood becomes 

 red in the arteries. The invariable result of this experiment seems to show that the 

 respiratory sense is connected with a supply of blood containing little oxygen and charged 

 with carbonic acid to the systemic capillaries by the arteries, and that it varies in intensity 

 with the degree of change in the blood. 



3. If, while artificial respiration is regularly performed, a large artery be opened and 

 the system be thus drained of blood, when the hemorrhage has proceeded to a certain 

 extent, the animal makes respiratory efforts, which become more and more violent, until 

 they terminate, just before death, in general convulsions. The same result follows when 

 the blood is prevented from getting to the system by applying a ligature to the aorta. 



These facts, which may be successively observed in a single experiment, remain pre- 

 cisely the same if we previously divide both pneumogastric nerves in the neck ; showing 

 that these are by no means the only nerves which convey the respiratory sense to the 

 medulla oblongata. 



The conclusions which may legitimately be drawn from the above-mentioned facts are 

 the following : 



The respiratory sense has its seat in the system and is transmitted to the medulla 

 oblongata by the general sensory nerves. It does not originate in the lungs, for it 

 operates when the lungs are regularly filled with pure air, if the system be drained of the 

 oxygen-carrying fluid. 



The respiratory sense is due to a want of oxygen on the part of the system, and not to 

 any fancied irritant properties of carbonic acid ; for, when the lungs are filled with air, 

 and the system is gradually drained of blood, although all the blood which finds its way 

 to the capillaries is fully oxygenated, as the quantity becomes insufficient to supply the 

 required amount of oxygen, the sense of want of air is felt, and respiratory efforts take 

 place. The experimental results on which these conclusions are based are invariable, 

 and we have demonstrated them repeatedly; so that the location of the respiratory 

 sense in the general system, and the fact that it is an expression of a want of oxygen, 

 seem as certain as that oxygen is taken up by the blood from the lungs and distributed 

 to the tissues by the arteries. With this view we can explain all the reflex phenomena 

 which are connected with the respiratory function. 



The supposition of Berard that the respiratory sense is due to distention of the right 

 cavities of the heart is disproved by the simple experiment of sudden excision of this 

 organ. In that case, as the system is drained of blood, efforts at respiration invariably 

 take place, though the supply of air to the lungs be continued. 



Sense of Suffocation. We must separate, to a certain extent, the respiratory sense 

 from the sense of distress from want of air, and its extreme degree, the sense of suffoca- 

 tion. The first is not a sensation, but an impression conveyed to the medulla oblongata, 

 giving rise to involuntary reflex movements. The necessities for oxygen on the part of 

 the system regulate the supply of air to the lungs. We have already seen that, once in 



