66 PHYSIOLOGY. 



the 1174 parts of oxygen absorbed, 1000 are again excreted as car- 

 bonic acid, leaving 174 parts, or nearly 15 percent, to be accounted 

 for. Some of this unites with the sulphur and phosphorusof the ori- 

 ginal components of the body ; the remainder most probably enters 

 into combination with the hydrogen of the fatty matter, thus forming 

 part of the water exhaled by the lungs. 



Of the nitrogen which enters so largely into the composition of 

 atmospheric air, a small portion only is absorbed, in consequence of 

 the difficulty with which it passes through animal membranes ; its 

 main use in the atmosphere being to dilute the oxygen. The exha- 

 lation and absorption of nitrogen appear usually to balance each 

 other, so that the amount of this gas in the respired air undergoes 

 little or no change. 



The quantity of carbonic acid exhaled varies at different times, 

 and under different circumstances. The mean of a great number 

 of observations gives about 160 grains of carbon per hour as the 

 quantity set free by an adult ; this would give 8 oz. Troy in the 24 

 hours. The amount varies with the development of the body, and 

 with the sex. In males, the quantity increases from eight years of 

 age till thirty, it remains stationary till forty, and then decreases 

 till old age, when it little exceeds that at ten. An extraordinary de- 

 velopment of the muscular system is always accompanied by the 

 extrication of a larger quantity ; the reverse is also true. 



In females there is the same proportional increase till puberty, 

 from which time it remains stationary during the menstrual life ; 

 after which it increases. After fifty, it decreases as in men. Du- 

 ring pregnancy the amount of exhalation is increased, and the same 

 takes place whenever the menses are temporarily arrested. 



The quantity exhaled is also increased by cold, exercise, or a full 

 meal, and by many of the exanthemata. It is diminished in typhus 

 fever, in chronic diseases of the respiratory organs, and in sleep. 



Independently of these variations, there is a difference in amount 

 in accordance with the time of day, being least at midnight, gradu- 

 ally increasing till midday, then again declining till midnight. 



The sources of the carbonic acid have been already adverted to. 

 It is not formed in the lungs as was originally supposed, but in the 

 tissues themselves, as is shown by the facts that venous blood con- 

 tains a larger amount of this gas than arterial ; and that an animal 

 placed in hydrogen, or nitrogen, still continues to evolve carbonic 

 acid. 



In regard to the change of colour effected by respiration, it must 

 be regarded as a purely physico-chemical phenomenon, inasmuch 

 as the same changes will take place in blood exposed to the air out 

 of the body, even through the medium of a thick membrane, such as 

 a bladder. The precise cause of the change of colour is as yet un- 

 settled. By Liebig it is supposed that the iron in the red corpuscles 



