RESPIRATION IN ANIMALS AND IN PLANTS. 183 



On Respiration. 



279. The concentration of the crude sap by the loss of its 

 superfluous fluid, and the occasional absorption of what may be 

 necessary to supply the amount insufficiently afforded by the 

 roots, are by no means the only functions of the leaves ; nor can 

 they be regarded as the most important. These organs supply 

 also the means of getting rid of a certain superfluous product, to 

 retain which within the system (at least in the form in which it 

 is set free) would be injurious and even destructive ; and they 

 serve the equally important purpose of introducing, from the air, 

 the element which chiefly gives firmness and solidity to the 

 vegetable tissue. 



280. It is well known that, when an animal is confined in a 

 limited quantity of air, it soon vitiates it, or renders it unwhole- 

 some ; so that free ventilation, by which the foul air is replaced 

 by that which is fresh, is one of the most important means of the 

 preservation of health. Now this change in the air is effected 

 by the removal of its oxygen, which is the element that chiefly 

 supports the life of all beings ; and by the substitution of car- 

 bonic acid gas set free from the lungs of the animal. Thus the 

 blood is purified by the removal of a noxious ingredient ; and is 

 rendered more capable of maintaining the life of the system, by 

 receiving one of the opposite character ; and this change is mani- 

 fested in its aspect, as well as in its properties, the dark purple 

 blood of the veins being converted, by exposure to the action of 

 the air in the lungs, into the bright scarlet fluid of the arteries. 

 (See ANIM. PHYSIOL. Chap, vi.) 



281. If the carbonic acid, which the blood takes up in its 

 passage through the vessels of the body, be not set free in this 

 manner, in consequence of any obstruction to the admission of 

 air into the lungs, or other similar cause, the animal dies. The 

 throwing-off this superfluous ingredient is, indeed, one of the 

 most constant of all the processes of the animal economy ; and 

 there is good reason that it should be so, since it is set at liberty 

 by the continual decay, to which all parts of the living body are 

 more or less subject (the softer ones, however, much more rapidly 



