INDEPENDENT VITALITY OF PARTS OF ORGANISM. 81 



as having a life of its own, which it maintains by drawing to 

 itself the nutrient material supplied by the circulating cur- 

 rent ; but as the continuance of its vital activity is dependent 

 upon the continuance of its nutrition, the life of no tissue 

 can be prolonged for any considerable period after the circu- 

 lation has ceased. But after the movement of 'the blood has 

 come to an end, though the body as a whole is dead, its part& 

 may remain alive for a certain time, and may perform their 

 functions, so long as they are supplied with the necessary 

 materials. Thus, various secretions, the growth of hair, and 

 muscular movements, have been observed to take place in 

 dead bodies. But they cannot continue, because the neces- 

 sary Conditions are withheld by the stoppage of the circu- 

 lation, a function which thus binds, as it were, into one 

 whole the scattered elements, and causes the different opera- 

 tions to minister one to another. As every component part 

 has an independent life, so has it a limited duration, quite 

 irrespective of that of the organism as a whole. Thus the 

 cells which float separately in the blood, seem to be con- 

 tinually undergoing change, dying, and giving place to new 

 ones. We have seen that the cells of the epidermis and of 

 some parts of the epithelium are being constantly thrown off 

 and renewed. The duration of the cells of fat and cartilage- 

 appears to be much greater; in fact, we have no precise- 

 knowledge of their term of life. That of the bony tissue is 

 probably greater still ; yet there is adequate evidence that 

 it is by no means indeterminate. But that of the muscular 

 and nervous tissues seems to depend almost entirely on the- 

 use that is made of them. Thus we may justly say, how- 

 ever startling the assertion may seem, that death and decay 

 are continually going on in every living animal body, and are 

 essential to the activity of its functions. 



66. Many animals are reduced to a state of apparent death 

 by dryness, by cold, or by exclusion of "the air. A curious 

 example of the first kind is furnished by the Tardigrada 

 (ZOOLOGY, 841) ; some species of which may not only be 

 completely dried up, but may even be exposed in that state 

 to a temperature much exceeding that of boiling water, 

 without losing the power of recovery when moistened. A 

 similar power of revival after being dried up is possessed by 

 the common Wheel Animalcule, and probably also by the 



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