532 OF RESPIRATION. 



diet has been changed, usually absorb oxygen until they are accustomed to their 

 new food. 1 Of the experiments made according to the second method, those of 

 M. Boussingault upon turtle-doves, and those of M. Barral upon the human 

 subject, appear to be trustworthy. The former states that the surplus of nitro- 

 gen in the food of the bird, above that excreted by the kidneys and intestinal 

 canal, is 2 grains daily; 3 whilst the latter gives the following as the results of 

 his observations upon himself and the other individuals already referred to 

 ( 566) :- 



Nitrogen in Food. Nitrogen excreted. 



A . 432.3 grs. 



B 327.3 " 



C 121.9 " 



D 421.5 " 



E 345.8 " 



In cases A, B, and E, the amount of Nitrogen which (being otherwise unac- 

 counted for) must be considered to have passed off by the lungs and skin, was 

 about l-75th of the oxygen consumed; a proportion which accords very well 

 with that deduced by MM. Regnault and Reiset from their experiments on 

 animals. In case D, however, it was only l-97th ; and in case c (that of a child 

 of six years old), it was as little as l-143d. It will be remembered that Nitro- 

 gen exists in an uncombined state in the blood ( 163) ; its percentage, how- 

 ever, is continually varying ; and no constant difference is observable between 

 the proportions yielded by arterial and venous blood respectively. 



[The alterations effected in the Blood by Respiration have already been fully 

 considered. See 163-166.] 



569. Exhalation and Absorption through the Lungs. The Air expired from 

 the lungs differs from that which was introduced into them, not merely in the 

 altered proportions of its Oxygen, Nitrogen, and Carbonic acid, but also in hav- 

 ing received (under ordinary circumstances at least) a large addition to its 

 watery vapor. This it doubtless acquires, in accordance with physical laws, 

 through its exposure to the warm blood which is spread out over a very exten- 

 sive surface, the intermediate membrane being extremely permeable ; and the 

 variations in its amount will depend upon the physical conditions under which 

 that exposure takes place. The air expired in ordinary respiration is charged 

 with as much watery vapor as saturates it at the temperature of the body ; and 

 consequently the amount of watery vapor thus exhaled, will vary (for equal 

 volumes of air at any given temperature) in the inverse proportion to that which 

 the air previously contained. But when the air is very cold and very dry, and 

 the respiration is unusually rapid, it may not remain sufficiently long in the air- 

 cells to be raised to the temperature of the body, or to be fully saturated with 

 moisture. The amount of watery vapor exhaled, moreover, will of course 

 depend in part upon the quantity of air which passes through the lungs. And 

 from these causes of variation, it happens that the amount, of watery vapor 

 exhaled in twenty-four hours ranges from about 6 to 27 oz. ; its usual range, 

 however, being between 16 and 20 oz. Of the fluid ordinarily exhaled with 

 the breath, a part doubtless proceeds from the moist lining of the nostrils, fauces, 

 &c. ; but it is indisputable that the greater proportion of it comes from the lungs, 

 since, when the respiration is entirely performed through a canula introduced 

 into the trachea, the amount of watery vapor which the breath contains is still 

 very considerable. Of the proper pulmonary exhalation, there can be no doubt 

 that the greater part is the mere surplus-water of the blood, and especially of 



1 "Ann. de Chim. et de Phys.," 1849. 2 "Comptes Rendus," 1846. 



