EXHALED FROM THE LUNGS. 117 



the cavity of the chest. It is, in fact, difficult to 

 suppose that the absorbents and lymphatics have 

 any peculiar tendency to absorb air, nitrogen, or hy- 

 drogen, and convey these gases into the circulation, 

 since the intestines, the stomach, and all spaces in 

 the body not filled with solid or liquid matters, con- 

 tain gases, which only quit their position when their 

 volume exceeds a certain point, and which, conse- 

 quently, are not absorbed. More especially in refer- 

 ence 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 passing 

 through the lungs, have become saturated with that 

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

 proportioned to its solvent power, like any other 

 liquid. By the respiratory motions all the gases 

 which fill the otherwise empty spaces of the body 

 are urged towards the chest ; for by the motion of 

 the diaphragm and the expansion of the chest a par- 

 tial vacuum is produced, in consequence of which 

 air is forced into the chest from all sides by the at- 

 mospheric pressure. The equilibrium is, no doubt, 

 restored, for the most part, through the windpipe, 

 but all the gases in the body must, nevertheless, 

 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 Jth be deducted as oxygen, there will still 



