26 THE FUNCTIONS OF ANIMALS. 
body, which make the organism aware of changes in its 
environment; and stimuli from within the body, which 
make it aware of the dispositions of its organs, eg. the 
stimuli transmitted by the afferent nerves of the muscles, 
tendons, etc. 
The chief functions of the nervous system are, then, to 
make the animal aware of its environment and to co- 
ordinate and integrate all its bodily functions and activities. 
As we ascend in the scale, we find that in addition the 
brain possesses, to an increasing extent, the power of 
correlating present and past experiences, and of originating 
or inhibiting action in accordance with this correlation. 
In whatever part there is activity, there is necessarily waste of 
complex substances and some degree of exhaustion; and it is 
interesting to notice, aS a triumph of histological technique, that 
Hodge, Gustav Mann, and others have succeeded in demonstrating in 
nerve cells the structural results (cellular collapse, etc.) of fatigue, and 
that in such diverse types as bee, frog, bird, and dog. 
Muscular activity—The movements of a unicellular 
animal are due to the contractility of the living matter, or 
of special parts of the cell, such as lashes or cilia. In 
sponges specially contractile cells begin to appear; in most 
higher animals such cells are aggregated to form the muscles. 
A piece of typical muscle consists of numerous fine 
transparent tubes or fibres, each invested by a sheath or 
sarcolemma, while the whole muscle is surrounded by 
connective tissue. It usually runs from one part of the 
skeleton to another, and is fastened to the skeleton by 
tendons or sinews. It is stimulated by motor nerves, and 
is richly supplied with blood. , 
When a muscle contracts, usually under a stimulus 
propagated along a motor nerve, there is of course a 
change of shape—it becomes shorter and broader. The 
source of the energy expended in work done is the 
“chemical explosion” which occurs in the fibres, for the 
oxygen stored up (intramolecularly) in the muscle enters 
into rapid union with carbon compounds. Heat, CO,, and 
water are produced as the result of this combustion, and 
lactic acid is also formed as a by-product. Besides the 
chemical change and the change of shape, there are also 
