THE RESPIRATORY SENSE. 157 



favor of the view that oxygen unites directly with carbonaceous matters in 

 the blood which it meets in the lungs, and by direct union with carbon, 

 forms carbon dioxide. 



That carbon dioxide makes its appearance in the blood itself, produced 

 in the red corpuscles, has been abundantly proved by observations already 

 cited, although it is impossible to determine to what extent this takes place 

 during life. It is likewise a product of the physiological wear of the tissues, 

 is absorbed by the blood circulating in the capillaries and is conveyed by the 

 veins to the right side of the heart. It has been shown that its production is 

 not immediately dependent upon the absorption of oxygen, for its formation 

 continues in an atmosphere of hydrogen or of nitrogen. It is most reason- 

 able to consider the carbon dioxide thus formed as a product of excretion. 

 The fact that it may easily be produced artificially, out of the body, does not 

 demonstrate that its formation in the body is as simple as when it is formed 

 by the process of combustion. It may be possible at some future time to 

 produce artificially all the excremetitious principles, as has already been done 

 in the case of urea ; but it can not be assumed that the mode of formation 

 of carbon dioxide, as one of the phenomena of nutrition, is precisely the same 

 as when it is made by chemical manipulations. 



THE KESPIKATOKY SEKSE. 



It is generally admitted that there exists in the system what may be re- 

 garded as a respiratory sense, which operates upon the respiratory nerve- 

 centre and gives rise to the involuntary movements of respiration ; and that 

 this sense is exaggerated by anything which interferes with respiration, and 

 is then conveyed to the brain, where it is appreciated as dyspnoea and finally 

 as the sense of suffocation. An exaggeration of the respiratory sense consti- 

 tutes a sense of oppression, which is referred to the lungs ; but it can not be 

 assumed, from sensations only, that the sense of want of air is really situated 

 in the lungs. 



At the present day it is hardly necessary to discuss the views of those 

 who attributed the sense of want of air, at least in its exaggerated form, 

 to an accumulation of carbon dioxide in the lungs (Marshall Hall), distention 

 of the right cavities of the heart (Berard), or to impressions conveyed to the 

 medulla oblongata, exclusively by the pneumogastric nerves. These theories 

 have long since been disproved and are now merely of historical interest. 

 Volkmann, in 1841, advanced the view that this sense is dependent upon a 

 deficiency of oxygen in the tissues, producing an impression which is con- 

 veyed to the medulla oblongata by the nerves of general sensibility. By a 

 series of experiments, this observer disproved the view that the respiratory 

 sense always originates in the lungs and is transmitted by the pneumogastrie 

 nerves ; and by exclusion, he located it in the general system. In a series of 

 experiments (Flint, 1861) the following facts, some of which had been previ- 

 ously noted, were observed : 



The chest was opened in a living animal, artificial respiration was care- 

 fully performed, inflating the lungs sufficiently but cautiously and taking 

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