112 SOURCES OF THE NITROGEN 



and thus reach the lungs, where they are exhaled ; but 

 the presence of membranes offers not the slightest .ob- 

 stacle to their passing directly into the cavity of the 

 chest. It is, in fact, difficult to suppose that the absor- 

 bents and lymphatics have any peculiar tendency to 

 absorb air, nitrogen, or hydrogen, and convey these 

 gases into the circulation, since the intestines, the stom- 

 ach, and all spaces in the body not filled with solid or 

 liquid matters, contain gases, which only quit their po- 

 sition when their volume exceeds a certain point, and 

 which, consequently, are not absorbed. More especial- 

 ly in reference to nitrogen, we must suppose that it is 

 removed from the stomach by some more direct means, 

 and not by the blood, which fluid must already, in pass- 

 ing through the lungs, have become saturated with that 

 gas, that is, must have absorbed a quantity of it, propor- 

 tioned to its solvent power, like any other liquid. By 

 the respiratory motions all the gases which fill the other- 

 wise empty spaces of the body are urged towards the 

 chest ; for by the motion of the diaphragm and the ex- 

 pansion of the chest a partial vacuum is produced, in 

 consequence of which air is forced into the chest from 

 all sides by the atmospheric pressure. The equilibrium 

 is, no doubt, restored, for the most part, through the 

 windpipe, but all the gases in the body must, neverthe- 

 less, receive an impulse towards the chest. In birds 

 and tortoises these arrangements are reversed. If we 

 assume that a man introduces into the stomach in each 

 minute only |th of a cubic inch of air with the saliva, 

 this makes in eighteen hours 135 cubic inches; and if 



