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 

 followed 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. This fact can be 

 very satisfactorily observed, as the diaphragm and other important respiratory muscles 

 are denuded and exposed to view. If the artificial respiration be interrupted or im- 

 perfectly performed, the animal almost immediately feels the want of air, and the exposed 

 respiratory muscles are thrown into violent but ineffectual 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, fiesoin de respirer, 

 which is conveyed to the respiratory nervous centre and gives rise to the ordinary reflex 

 and involuntary movements of respiration, that this sense is exaggerated by any thing 

 which interferes with respiration, and is then carried on to the brain, where it is 

 appreciated as dyspnoaa and finally as the overpowering sense of suffocation. An 

 exaggeration of the respiratory sense constitutes an oppression, which is referred to the 

 lungs. It has been demonstrated, however, that the sensation of hunger, which is felt 

 in the stomach, and of thirst, which is felt in the throat and fauces, have their seat 

 really in the general system, and are instinctively referred to the parts mentioned, 

 because they are severally relieved by the introduction of food into the stomach and the 

 passage of liquid along the throat and oesophagus. It cannot, therefore, 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 interesting of the physio- 

 logical points connected with respiration. 



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

 located 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 incon- 

 sistent with such an exclusive view. 



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

 oxygenated 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 obstruc- 

 tion to the passage of blood through the lungs. John Reid thought it was due in a 

 measure to the circulation of venous blood in the medulla oblongata. What has been 

 shown to be the correct explanation was given by Yolkmann in 1841. He regarded the 

 sense of want of air as dependent on 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 this sense 

 resides in the lungs and is transmitted along the pneumogastric nerves ; and, by exclusion, 

 he located it in the general system and showed that such a supposition is sufficient to 

 explain all the phenomena connected with the respiratory movements. In the hope of 



