INDEPENDENT VITALITY OF PARTS OF ORGANISM. Si 
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 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 
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 deqth 
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 
G 
