32 THE FUNCTIONS OF ANIMALS. 
physical osmosis; it is modified by the fact that the cells 
are living. : 
But though we cannot analyse living matter, nor thoroughly 
explain the changes by which the material of the body 
breaks down or is built up, we can trace, by chemical 
analysis, how food passes through various transformations 
till it becomes a usable part of the living body, and we can 
also catch some of the waste products formed when muscles 
_or other parts are active. 
What is known in regard to the structure of protoplasm does not 
help the physiologist very much. The microscopists discover an in- 
tricate structure which pervades cach unit of living matter, but no 
physiologist dreams of explaining the life of a cell in terms of its 
microscopically visible structure. 
One general idea, however, the study of structure has suggested, 
which the conclusions of physiologists corroborate. This idea is—that 
a cell consists of a relatively stable living framework, and of a changeful 
content enclosed by it. 
Now, many physiologists regard the framework as the genuine living 
protoplasm, and the content as the material upon which it acts. ‘‘The 
framework is the acting part, which lives, and is stable ; the content is 
the acted-on part, which has never lived, and is labile, that is,—in a 
state of metabolism or chemical transformation.” This view naturally 
leads those who adopt it to regard protoplasm as a sort of ferment 
acting on less complex material which is brought to it, which forms 
the really changeful part of each cell. 
Somewhat different, however, is another idea,—that the protoplasm 
is itself the seat of constant change ; that it is constantly being unmade 
and remade. On the one hand, more or less crude food passes into 
life by an ascending series of assimilative or constructive chemical 
changes, with each of which the material becomes molecularly more 
complex and more unstable. On the other hand, the protoplasm, as 
it becomes active or a source of energy, breaks down in a descending 
series of disruptive or destructive chemical changes ending in waste 
products. ° 
The former view, which considers protoplasm as a sort of ferment, 
restricts the metabolism to the material on which the protoplasm acts. 
The second view regards protoplasm as the climax or central term of 
the constructive and disruptive metabolism. 
It is highly probable that there is no one substance which should be 
called protoplasm, but that vital phenomena depend upon the inter- 
actions of several complex substances. As Verworn says, ‘‘ The life- 
process consists in the metabolism of proteids.” 
Generalising from his studies on colour sensation, Professor Hering 
was led to regard all life as an alternation of two kinds of activity, 
both induced hy stimulus, the one tending to storage, construction, 
assimilation of snaterial, the other tending to explosion, disruption, 
disassimilation. 
