66 THE CELL 



continued long enough, the excitability may be entirely destroyed. All these 

 phenomena which are best known in man from his subjective experience of 

 the results of severe muscular work are included under the term fatigue. 



Now it has been shown that fatigue, for the most part at least, is caused 

 by the products formed in metabolism (J. Eanke), as may be seen from 

 such facts as the following: if the blood of an exhausted dog be injected into 

 a vein of a fresh dog, the latter immediately exhibits very evident signs of 

 fatigue. 



If a fatigued organ be allowed to rest for a long time a remarkable thing 

 occurs : the organ completely recovers i. e., its former functional power has 

 returned. This is not difficult to explain for the free-living unicellular 

 organisms; for they give off the decomposition products to the surround- 

 ing medium in a very simple manner. And even in the higher animals the 

 recovery after fatigue presents no great difficulties, for the waste products 

 are carried away from the organ by the circulation of the blood and lymph, 

 and at the same time the blood places new nutrient material at its disposal. 

 The same phenomenon is observed however in the organs of cold-blooded 

 animals which have been cut out of the body. A frog's muscle which by 

 repeated stimulation has been brought to a condition where it cannot contract 

 at all, recovers and becomes functional again in spite of the fact that there is 

 no circulation to carry away waste products. It follows therefore that recov- 

 ery is not conditioned solely upon the removal of waste products, but that 

 other factors also must be taken into account, with which we are not yet thor- 

 oughly acquainted. 



4. DEATH 



We have seen that the most widely different external agencies of suffi- 

 cient intensity and sufficient duration have the power to check life and to 

 bring on death. Changes also which are going on in the cells without any 

 such external influences can reduce their functional powers. In the course 

 of life these alterations come on gradually, in some beings more rapidly than 

 others, but always inevitably. They are known by the term senescence. If 

 they progress far enough, death ensues as the result of old age. This form 

 of death is, in man at least, only rarely to be considered ; for the body is sub- 

 jected to many external accidents of all kinds, and only in the most excep- 

 tional cases does it escape all of them. The senescent changes, however, play 

 an important role even here, for by their influence the power of resistance of 

 the organism to the accidents which it must encounter is more and more 

 reduced. 



After death the body is destroyed as a rule within a short time, partly by 

 autolysis of the organs (cf. page 38), partly by processes of decomposition and 

 putrefaction which are carried on by the lowest organisms. The carbon and 

 hydrogen of the body pass off in the atmosphere as carbon dioxide and water 

 vapor; and the nitrogen and sulphur, after a series of transformations taking 

 place under the influence of Bacteria, are combined with metals in the form of 

 nitrates and sulphates which are taken up by the water of the soil. These sub- 

 stances, carbon dioxide, water, nitrates and sulphates are the normal food of 

 green plants, and by the synthetic processes going on within them are combined 



