THE CELL. 4 
gations of cells known as tissues. The greater part of a tree 
may be said to be made up of the thickened walls of the cells, 
and these are destitute of true vitality, unless of the lowest 
order ; while the really active, growing part of an old and large 
tree constitutes but a small and limited zone, as may be learned 
from the plates of a work on modern botany tepresenting sec- 
tions of the wood. 
Animals, too, have their rigid parts, in the adult state espe- 
cially, resulting from the thickening of a part or the whole of 
the cell by a deposition usually of salts of lime, as in the case of 
the bones of animals. But in some cases, as in cartilage, the 
cell wall or capsule undergoes thickening and consolidation, 
and several may fuse together, constituting a matrix, which is 
also made up in part, possibly, of a secretion from the cell pro- 
toplasm. In the outer parts of the body of animals we have a 
great abundance of examples of thickening and hardening of 
cells. Very well known instances are the indurated patches of 
skin (epithelium) on the palms of the hands and elsewhere. 
It will be scarcely necessary to remark that in cells thus 
altered the mechanical has largely taken the place of the vital 
in function. This at once harmonizes with and explains what 
is a matter of common observation, that old men are less active 
—have less of life within them, in-a word, than the young. 
Chemically, the cellulose wall of plant-cells consists of carbon, 
hydrogen, and oxygen, in the same relative proportion as exists 
in starch, though its properties are very different from those of 
that substance. 
Turning to cell contents, we find them everywhere made up 
of a clear, viscid substance, containing almost always granules 
of varying but very minute size, and differing in consistence, 
not only in different groups of cells, but often in the same cell, 
so that we can distinguish an outer portion (ectoplasm) and an 
inner more fluid and more granular region (endoplasm). 
The nucleus is a body with very clearly defined outline (in 
_ some cases limited by a membrane), through which an irregu- 
lar network of - fibers extends that stains more deeply than any 
other part of the whole cell. 
Owing to the fact that it is so readily changed by the action 
of reagents, it is impossible to ascertain the exact chemical com- 
position of living protoplasm; in consequence, we can only 
infer its chemical structure, etc., from the examination of the 
dead substance. 
In general, it may be said that protoplasm belongs to the 
