INDEPENDENT VITALITY OF PARTS OF ORGANISM. 8i 
| as having a life of its own, which it maintains by drawing to 
itself the nutrient material supplied by the circulating cur- 
tent ; 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 parts 
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 
“respective of that of the organism asa whole. Thus the 
‘tells 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 
snowledge of their term of life. That of the bony tissue is 
robably greater still ; yet there is adequate evidence that 
‘itis by no means indeterminate. But that of the muscular 
and nervous tissues seems to depend almost entirely on the 
se 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 
dryness, by cold, or by exclusion of*the air. A curious 
ple of the first kind is furnished by the Tardigrada 
fooLoGy, § 841); some species of which may not only be 
pletely dried up, but may even be exposed in that state 
y a temperature much exceeding that of boiling water, 
t losing the power of recovery when moistened. A 
power of revival after being dried up~is possessed by 
common Wheel = and probably also by the 
Jf a 
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