114 USE OF THE SALIVA. 



certain graminivorous animals, has plainly for one 

 object a renewed and repeated introduction of oxy- 

 gen ; for a more minute mechanical division of the 

 food only shortens the time required for solution. 



The unequal quantities of air which reach the 

 stomach with the saliva in different classes of ani- 

 mals explain the accurate observations made by 

 physiologists, who have established beyond all doubt 

 the fact, that animals give out pure nitrogen through 

 the skin and lungs, in variable quantity. This fact 

 is so much the more important, as it furnishes the 

 most decisive proof, that the nitrogen of the air is 

 applied to no use in the animal economy. 



The fact that nitrogen is given out by the skin 

 and lungs, is explained by the property which animal 

 membranes possess of allowing all gases to permeate 

 them, a property which can be shewn to exist by the 

 most simple experiments. A bladder, filled with 

 carbonic acid, nitrogen, or hydrogen gas, if tightly 

 closed and suspended in the air, loses in twenty-four 

 hours the whole of the enclosed gas ; by a kind of 

 exchange, it passes outwards into the atmosphere, 

 while its place is occupied by atmospherical air. A 

 portion of intestine, a stomach, or a piece of skin 

 or membrane, acts precisely as the bladder, if filled 

 with any gas. This permeability to gases is a me- 

 chanical property, common to all animal tissues; 

 and it is found in the same degree in the living as 

 in the dead tissue. 



It is known that in cases of wounds of the lungs 



