SECRETION AND EXCRETION. 183 



GENERAL SUMMARY OF THE EXCRETING PROCESSES. 



We have now passed in review the various processes, by which 

 the products of the disintegration or the animal tissues are carried 

 off ; and we have seen that the necessity for their removal is much 

 more urgent than for their replacement. A cold-blooded animal 

 may subsist for some weeks, or even months, without a fresh supply 

 of food ; the waste of its tissues being so small, if it remain in a 

 state of rest, as to be quite compatible with the continuance of its 

 life ; and a warm-blooded animal may live for many days, or even 

 weeks, provided that it has in its body a store of fat sufficient to 

 keep up its heat by the combustive process. But in either case, if 

 the exhalation of carbonic acid by the lungs, the elimination of 

 biliary matter by the liver, the separation of urea or uric acid by the 

 kidneys, or the withdrawal of putrescent matter by the intestinal 

 glandules, be completely checked, a fatal result speedily ensues ; 

 more speedily in warm-blooded animals than in those which cannot 

 sustain a high independent temperature, on account of the greater 

 proneness to decomposition in the bodies of the former, than in 

 those of the latter ; and more speedily in the latter, when their 

 bodies are kept at an elevated temperature by the warmth of the 

 surrounding medium, than when the degree of heat is so low, that 

 there is little proneness to spontaneous change in the substance of 

 their bodies. 



It may be taken as a general principle, in regard to the excreting 

 processes (including respiration), that they have a three-fold pur- 

 pose ; in the first place, to carry off the normal results of the waste 

 or disintegration of the solid tissues, and of the decomposition of 

 the fluids ; in the second place, to draw off the superfluous ali- 

 mentary matter, which, though received into the circulating current, 

 is not converted into solid tissue, in consequence of the want of 

 demand for it ; and in the third place, to carry off the abnormal 

 products, which occasionally result from irregular or morbid changes 

 in the system. Thus by the lungs are excreted a large amount of 

 carbon, and some hydrogen, resulting from the disintegration of the 

 tissues, especially the nervous and muscular ; the same elements, 

 in animals that take in a large proportion of farinaceous or oleagi- 

 nous aliment, may be derived immediately from the food, without 

 any previous conversion into solid tissue ; and there can be little 

 doubt that the respiratory function is also an important means of 

 purifying the blood from various deleterious matters, either intro- 

 duced from without (such as narcotic poisons), or generated within 



