22 ORGANIC BEHAVIOUR 
We must not press the explosion analogy too far. The 
essential thing seems to be that the protoplasm of the cell has 
the power of building up complex and unstable chemical 
compounds, which are perhaps stored in its spongy sub- 
stance; and that these unstable compounds, under the 
influence of a stimulus (or, possibly, sometimes spontaneously), 
break down into simpler and more stable compounds. In the 
case of muscle-cells, this latter change is accompanied by an 
alteration in length of the fibres, and consequent movements 
in the animal, the products of the disruptive change being 
useless or harmful, and being, therefore, removed as soon as 
possible. But very frequently the products of explosive activity 
are made use of. In the case of bone-cells, one of the products 
of disruption is of permanent use to the organism, and con- 
stitutes the solid framework of the skeleton. In the case of 
the secreting cells in the salivary and other digestive glands, 
some of the disruptive products are of temporary value for 
the preparation of the food. It is probable that these useful 
products of disruption, permanent or temporary, took their 
origin in waste products for which natural selection has found 
a use, and which have been gradually rendered more and more 
efficacious in modes of organic behaviour increasingly complex. 
In the busy hive of cells which constitutes what we call 
the animal body, there is thus ceaseless activity. During 
periods of apparent rest the protoplasm is engaged in con- 
structive work, building up fresh supplies of unstable materials, 
which, during periods of apparent activity, break up into 
simpler and more stable substances, some of which are useful 
to the organism, while others must be got rid of as soon 
as possible. From another point of view, the cells during 
apparent rest are storing up energy to be utilized by the 
animal during its periods of activity. The storing up of 
available energy may be likened to the winding up of a watch 
or clock; it is when an organ is at rest that the cells are 
winding themselves up; and thus we have the apparent 
paradox that the cell is most active and doing most work 
when the organ of which it forms a part is at rest. During 
