THE RESPIRATORY SENSE. 1 65 



of breathing takes place without our knowledge ; but even when the air is but little 

 vitiated, when its entrance into the lungs is slightly interfered with, or when a consider- 

 able portion of the pulmonary structure is involved in disease, we experience a certain 

 sense of uneasiness and become conscious of the necessity of respiratory efforts. This 

 gradually merges into the sense of suffocation, and, if the obstruction be sufficient, is fol- 

 lowed by convulsions, insensibility, and finally by death. 



Although we are not sensible of any want of air under ordinary conditions, it was 

 proven by the celebrated experiment of Robert Hook, in 1664, that there is a want 

 always felt by the system, and that, if this want be effectually supplied, no respiratory 

 movements will take place. We have often repeated the experiment demonstrating this 

 fact. If a dog be brought completely under the influence of ether, the chest and abdo- 

 men opened, and artificial respiration be carefully kept up by means of a bellows fixed in 

 the trachea, even after the animal has come from under the influence of the anaesthetic so 

 as to look around and wag his tail when spoken to, he will frequently cease all respiratory 

 movements when the air is adequately supplied to the lungs ; but if the artificial respi- 

 ration be interrupted or imperfectly performed, the animal almost immediately feels the 

 want of air, and the respiratory muscles are thrown into violent contraction. 



It is generally admitted, indeed, that there exists in the system what may appropri- 

 ately be regarded as a respiratory sense, or, as it is called by the French, besoin de respirer, 

 which operates upon the respiratory nervous centre and gives rise to the involuntary 

 movements of respiration, and that this sense is exaggerated by any thing which inter- 

 feres with respiration, and is then conveyed to the brain, where it is appreciated as 

 dyspnoea and finally as the overpowering sense of suffocation. An exaggeration of the 

 respiratory sense constitutes a sense of oppression, which is referred to the lungs ; but it 

 cannot be assumed, from sensations only, that the sense of want of air is really situated 

 in the lungs. The question of its seat and its immediate cause is one of the most inter- 

 esting of the physiological points connected with respiration. 



Many physiologists accept the view of Marshall Hall, that the respiratory sense has its 

 origin in the lungs, is carried to the medulla oblongata by the pulmonary branches of the 

 pneumogastric nerves, and is due to the accumulation of carbonic acid in the pulmonary 

 vesicles; but there are facts in physiology and pathology which are inconsistent with 

 such an exclusive view. 



In cases of disease of the heart, when the system is imperfectly supplied with oxygen- 

 ated blood, the sense of suffocation is frequently most distressing, although the lungs be 

 unaffected and receive a sufficient supply of pure air. This and other similar facts led 

 Berard to adopt the view that the respiratory sense has its point of departure in the right 

 cavities of the heart and is due to their distention as the result of obstruction to the pas- 

 sage of blood through the lungs. John Reid thought it was due in a measure to the cir- 

 culation of venous blood in the medulla oblongata. Volkmann, in 1841, advanced the 

 view that the sense of want of air is dependent upon a deficiency of oxygen in the tissues, 

 producing an impression which is conveyed to the medulla oblongata by the nerves of 

 general sensibility. By a series of experiments, this observer disproved the view that 



I this sense resides in the lungs and is transmitted along the pneumogastric nerves ; and, 

 by exclusion, he located it in the general system. In the hope of settling some of these 

 questions, we instituted, in 1861, 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 per- 

 formed, 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 

 ivsj'irutory 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 



