388 OF RESPIRATION. 



700. Thus it is evident, that a part of the change effected in the 

 Blood consists in an alteration in the proportion of the gases which 

 always exist in it, either entirely free, or in a state of such loose com- 

 bination that they can be removed by the air-pump. But it may be 

 suspected, that a portion of the effect consists in the oxidation of t"he 

 proteine of the fibrinous constituent ; since the fibrine of arterial blood 

 possesses properties that distinguish it from that of venous. It has 

 been usually supposed that the hematosine of the red corpuscles under- 

 goes a change under the influence of oxygen in the lungs, and a con- 

 verse change in the systemic capillaries, where it is subjected to the 

 influence of carbonic acid ; this change being indicated by the altera- 

 tion in the colour of the red corpuscles. The alteration in question, 

 however, seems due rather to a physical than to a chemical change 

 ( 222) ; and we have no direct evidence, though much that is indirect, 

 of the special influence of the aeration of the blood upon the contents 

 of the red corpuscles. It appears tolerably certain that a part of the 

 oxygen imbibed in the lungs, is appropriated to the oxidation of the 

 matters set free by the decomposition of the solid tissues ; whilst 

 another part enters into combination with fatty, saccharine, farina- 

 ceous, and other matters, existing in the blood itself, and destined to be 

 carried off in the form of carbonic acid and water, without ever enter- 

 ing into the composition of the solid fabric. The relative amounts 

 of carbonic acid formed in these two modes, vary in different animals 

 and in different states of the same individual : for a man in a warm 

 atmosphere, taking a moderate amount of exercise, may thus set free, by 

 the waste of his muscular and other tissues, a sufficient quantity of carbon 

 for the maintenance of his animal heat by its union with oxygen ; but this 

 is far from being sufficient, when a larger amount of heat must be 

 evolved, to sustain the temperature of the body in a colder climate. 



701. The blood parts in the lungs with a very large amount of mois- 

 ture ; for the inspired air is always saturated with fluid, as soon as it 

 reaches the air-cells ; and, as it is heated at the same time to about 98, 

 it thus receives a considerable addition, even if it were previously 

 charged with as much as it could contain at a lower temperature. The 

 total quantity of fluid thus disengaged will vary, therefore, with the 

 amount previously contained in the atmosphere, being greater as this 

 was less, and vice versd; but the quantity that usually passes off seems 

 to be from 16 to 20 ounces in the twenty-four hours. It cannot be 

 doubted, that a great part of this water is a simple exhalation of that 

 which has been absorbed ; but, on the other hand, it seems probable 

 that a portion of it may be actually formed in the system, by the 

 union of a portion of the oxygen absorbed in the lungs, with the 

 hydrogen of the combustible matters of the blood. In the various 

 forms of saccharine and farinaceous aliments, the proportion of hydrogen 

 and oxygen are such as would of themselves form water, when the 

 carbon is withdrawn ; but in oily and fatty matters, the proportion of 

 oxygen is far too small thus to neutralize the hydrogen ; and it seems 

 likely that by their oxidation in the blood, as by their combustion else- 

 where, water is actually generated by the union of atmospheric oxygen 



