400 ON RESPIRATION. 



II. Chemical Phenomena of Respiration. 



533. We naturally pass from the foregoing inquiries to those that relate to 

 the alterations in the air, which are effected by Respiration. It was formerly 

 supposed that the blood arrived at the lungs charged with Carbon, that this 

 carbon was united in their cells to the Oxygen of the atmosphere, and that 

 in this manner a certain amount of the Oxygen of the inspired air was being 

 continually converted into Carbonic acid, which thus replaced it in the expired 

 air. Subsequent researches, however, appear to have satisfactorily proved, 

 that this is not a true account of the changes which take place in the Lungs ; 

 and that it would be more correct to say, that the blood comes to the lungs 

 charged with Carbonic acid, formed by the union of carbon and oxygen in the 

 systemic capillaries ; this it imparts to the inspired air, at the same time 

 abstracting from it a volume of Oxygen which is always as large, and usually 

 greater. Hence it is not correct to speak of a certain quantity of the inspired 

 oxygen as being converted into carbonic acid in the lungs ; but it should rather 

 be said, that a certain quantity of oxygen is absorbed, and a certain quantity 

 (generally less than the equivalent bulk) of carbonic acid exhaled. The pro- 

 portion of these quantities is by no means constant ; varying with different 

 species, and with the same animal at different ages and at different periods of 

 the year. According to Dr. Edwards, the quantity of oxygen which entirely 

 disappears from the air is sometimes as much as one-third of the whole ; it is 

 greatest in the young animal, and is sometimes almost imperceptible in the 

 adult. It appears probable that a part of this Oxygen is made to combine with 

 Hydrogen set free in the systemic capillaries ; and that the water thus gene- 

 rated forms part of that exhaled from the lungs. A sort of combustion of 

 Hydro-Carbon thus appears to be continually going on in the body at large, the 

 products of which are got rid of by the lungs; and this process is mainly, if 

 not solely, instrumental in the maintenance of Animal Heat. (See Chap, xm.) 



534. The quantity of Carbonic Acid excreted by the lungs has been esti- 

 mated by some experimenters at as much as 39,600 cubic inches in twenty- 

 four hours; this amount of gas would contain 5,148 grains, or 11 ounces 

 (Troy) of solid carbon. This estimate was formed upon the results of an expe- 

 riment continued during a short time ; in which, from the nature of the appa- 

 ratus employed, the respirations were to a certain degree laborious, and the 

 quantity of air renewed at each movement was therefore greater ; and there 

 are several reasons for regarding it as much too high. In Mr. Coathupe's 

 experiments, great care was taken to render the inspiration as free as possible 

 from effort ; and the measuring process was continued for a much longer time. 

 According to his statement, the. quantity of Carbonic acid generated in twenty- 

 four hours is about 17,856 cubic inches ; this will contain 2,616 grains, or 5| 

 ounces of solid carbon, a quantity, which we may very well imagine to be 

 thus excreted, and which corresponds very closely with the results obtained 

 by MM. Prevost and Dumas.* The proportion of carbonic acid contained in 



results of the examinations are so nearly uniform that disease may be suspected in any 

 man who cannot blow out nearly so many cubic inches as the average of those of ihe 

 same height, even when, by external measurement, his chest appears to be of full size. 

 Indeed, in general, the size of the chest affords no good indication of the capacity of expi- 

 ration. The only exceptions among healthy men" to the general rule of the direct pro- 

 portion between the height of the body and the capacity, are in the cases of fat men 

 whose capacity is always low. M. C.] ' 



* M M. Andral and Gavarret state the following as the results of experiments made on 

 seventy-two persons (thirty-six males and thirty-six females), to determine the quantity 

 of carbonic acid exhaled in breathing. These experiments were made as nearly as pos- 

 sible under the same circumstances, as regards health, time of the day, amount of exer- 



