FOODOF ANIMALS. 191 



There are also in the blood some colorless globules, which I will 

 speak of hereafter. 



It now remains to point out the uses of the several ingredients 

 of the blood. 



To explain the use of the red disks, it is necessary to premise 

 that the decomposition of the tissues, which constitutes one element 

 of the nutritive act, requires the presence of oxygen. Now the of- 

 fice of these disks is to absorb oxygen from the atmosphere, during 

 their passage through the pulmonary capillaries, and carry it to the 

 other system of capillary vessels, which permeate all parts of the 

 body. Here the oxygen combines with the living tissues, and the 

 result of its combination with their elements is the production of 

 carbonic acid and of the elements of the bile and urine. This 

 carbonic acid thus formed, again combines with the red disks 

 which have just parted with their oxygen, and is carried by them 

 back to the pulmonary capillaries, where it is thrown off with the 

 expired air. 



The office of the red disks is thus twofold ; 1st, to carry oxygen 

 from the pulmonary capillaries to the general capillary system, 

 where the oxygen combines with the tissues and promotes their de- 

 bomposition ; and 2d, to carry the carbonic acid here generated back 

 to the pulmonary capillaries, where it is rejected from the body. 



One word as to the mode in which the remaining products of 

 ihe decomposition of the tissues are disposed of. The bile con- 

 .ains a large portion of carbon in combination with hydrogen, but 

 IS this is re-absorbed after serving its purpose in digestion, it is not 

 properly an excretion. The carbon and hydrogen it contains are 

 iltimately discharged by the lungs in the form of carbonic acid and 

 A'ater. 



The elements of urine contain the nitrogenized products of the 

 jlecomposition of the tissues, chiefly in the form of urea, which 

 leadily passes into carbonate of ammonia, by exposure to the air. 



We have now explained that portion of the nutritive act, which 

 lonsists in the decomposition of the tissues. Oxygen is brought 

 y the red disks; this causes their decomposition; the results of 

 tiis decomposition are : 1st, carbonic acid, which is carried back 

 3 the lungs by the red disks, and then thrown off; 2nd, urea 

 Ind lithic acid, which are compounds of nitrogen and the other 

 Icments of the tissues, and which are rejected by the kid- 



