THE ANIMAL BODY. ov 
ive when the reverse process takes place. The results of this 
process are eliminated as excreta, or useless and harmful prod- 
ucts. Since all the vital activities of cells can only be mani- 
fested when supplied with food, it follows that living organisms 
convert potential or possible energy into kinetic or actual en- 
ergy. When lifeless, immobile matter is taken in as food and, 
as a result, is converted by a process of assimilation into the 
protoplasm of the cell using it, we have an example of poten- 
tial being converted into actual energy, for one of the proper- 
ties of all protoplasm is its contractility. Assimilation implies, 
of course, the absorption of what is to be used, with rejection 
of waste matters. : 
The movements of protoplasm of whatever kind, when due 
to a stimulus, are said to indicate irritability ; while, if inde- 
pendent of any external source of excitation, they are denomi- 
nated automatic. 
Among agents that modify the action of all kinds of proto- 
plasm are heat, moisture, electricity, light, and others in great 
variety, both chemical and mechanical. It can not be too well 
remembered that living things are what they are, neither by 
virtue of their own organization alone nor through the action 
of their environment alone (else would they be in no sense dif- 
ferent from inanimate things), but because of the relation of 
the organization to the surroundings. 
Protoplasm, then, is contractile, irritable, automatic, absorp- 
tive, secretory (and excretory), metabolic, and reproductive. ; 
But when it is affirmed that these are the fundamental prop- 
erties of all protoplasm, the idea is not to be conveyed that cells 
exhibiting these properties are identical biologically. No two 
masses of protoplasm can be quite alike, else would there be no 
distinction in physiological demeanor—no individuality. Livery 
cell, could we but behold its inner molecular mechanism, differs 
from its neighbor. When this difference reaches a certain de- 
gree in one direction, we have a manifest differentiation leading 
to physiological division of labor, which may now with advan- 
tage be treated in the following section. 
THE ANIMAL BODY. 
An animal, as we have learned, may be made up of a single 
cell in which each part performs much the same work; or, if 
there be differences in function, they are ill-defined as compared 
