164 EESPIEATION. 



it circulates ; but the avidity of the tissues for oxygen and the readiness with which 

 they exhale carbonic acid leave no room for doubt that much of this change is effected 

 in their substance. 



Oxygen, carried by the blood to the tissues, is appropriated and consumed in their 

 substance, together with the nutritive materials with which the circulating fluid is 

 charged. We are acquainted with some of the laws which regulate its consumption but 

 have not been able to follow it out and ascertain the exact nature of the changes which 

 take place. All that we can say definitely on this point is, that it unites with the organic 

 principles of the system, satisfying the "respiratory sense" and supplying an imperative 

 want which is felt by all animals and which extends to all parts of the organism. After 

 being absorbed, it is lost in the intricate processes of nutrition. There is no evidence in 

 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 carbonic acid. 



2. How is carbonic acid produced by the system? That carbonic acid makes its 

 appearance in the blood itself, produced in the red corpuscles, has been abundantly proven 

 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 decomposition of 

 the tissues, whence it is absorbed by the blood circulating in the capillaries and conveyed 

 by the veins to the right side of the heart. It has been experimentally demonstrated 

 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 reasonable 

 to consider the carbonic acid thus formed as a product of excretion or disassimilation, 

 like urea, creatine, or cholesterine. 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. We may be able at some future time to pro- 

 duce artificially all the excrementitious principles, as has already been done in the case 

 of urea; but we are hardly justified in supposing that the mode of formation of carbonic 

 acid, as one of the phenomena of nutrition, is precisely the same as when it is made by 

 our chemical manipulations. 



As expressing nearly all that is known, even at the present day, regarding the mode 

 of formation of carbonic acid in the economy, we may take the following concluding 

 passage from the paper of Collard de Martigny, published in 1830 : 



" The carbonic acid expired is a product of assimilative decomposition, secreted in 

 the capillaries and excreted by the lungs." 



The carbonic acid thus produced is taken up by the blood, part of it in a free state in 

 solution, particularly in the plasma, and a part which has united with the carbonates to 

 form bicarbonates. Carried thus to the lungs, the free gas is removed by simple dis- 

 placement, and that which exists in combination is set free by the acids found in the 

 pulmonary substance. 



3. What is the nature of the intermediate processes, from the disappearance of 

 oxygen to the evolution of carbonic acid? A definite answer to this question would 

 complete our knowledge of the respiratory process ; but this, in the present state of the 

 science, we are not prepared to give. We can only repeat what has already been so 

 frequently referred to, that oxygen must be considered as a nutritive principle, and 

 carbonic acid, as a product of excretion. The intermediate processes belong to the general 

 function of nutrition, with the intimate nature of which we are unacquainted. We 

 have not sufficient evidence for supposing that this process is identical witli what is 

 generally known as combustion. 



The Respiratory Sense, or Want on the part of the System which induces the 

 Respiratory Movements. (Besoin de respirer.) 



We are all familiar with the peculiar and distressing sense of suffocation which 

 attends an interruption in the respiratory process. Under ordinary conditions, the act 



