OF ORGANIZED BEINGS. 411 



It is the carbon which makes the venous blood, or that 

 which circulates in the veins dark, and when it is de- 

 prived of it, it becomes light red or arterial blood. 



We have observed that only some of the higher classes 

 of animals and men were furnished with proper lungs, 

 but we have endeavored to show at the same time, that 

 air was as absolutely necessary for the support of one or- 

 ganized being as another, as necessary for plants as 

 animals, although the organs through which they receive 

 it may materially differ. 



We have thus described two functions that belong to 

 all organized living beings, viz. that of assimilation or 

 digestion and respiration. We now come to a third, 

 which is called the circulatory function, which consists 

 in the circulation of the fluids through the body. The 

 circulatory function is carried on by means of vessels ar- 

 ranged in proper order. We have in part been obliged 

 to anticipate this subject in order to convey a general 

 idea of the importance of atmospheric air. 



The circulatory function in man includes those vessels 

 that convey the blood, which consist of a heart, arteries 

 and veins. The heart is the central organ from which 

 the blood is thrown and to which it returns. In man it 

 has four cavities, two of which are called ventricles, and 

 two auricles. The two auricles occupy the base or the 

 superior and posterior portion, and the ventricles the in- 

 ferior. On each side the auricle corresponds with its 

 corresponding ventricle. In the right cavities are con- 

 tained the dark or venous blood which is to be sent to the 

 lungs, there to be submitted to the action of the air, and 

 in the left the light or arterial blood which has undergone 

 this action and which is to be sent to all parts of the 

 body. The former receive therefore the blood from all 

 parts of the body, and by contracting propel it to the 

 lungs ; the latter receive it from the lungs and send it to 

 all parts of the body. Those vessels that convey the blood 

 from the heart are called arteries, and those that return 

 it to the heart are called veins. 



In fish the heart has only two cavities, one auricle and 

 one ventricle. The blood is received from all parts of 

 the body into the auricle, this throws it into the ven- 



