26 THE FUNCTIONS OF ANIMALS. 



There are two chief kinds of stimuli which are transmitted 

 to the central nervous system stimuli from without the body, 

 which make the organism aware of changes in its environ- 

 ment \ and stimuli from within the body, which make it 

 aware of the dispositions of its organs, e.g. the stimuli trans- 

 mitted 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, it and each of its fibres 

 becomes shorter and broader. The contraction of the 

 fibre is in itself a physical rather than a chemical phen- 

 omenon, like a change in the state of a spiral spring. In 

 the actual contraction there is no using up of oxygen or 

 output of CO 2 , but lactic acid is dissociated from the 

 muscle-substance. To make another contraction possible 

 lactic acid must be re-introduced, and this process requires 

 the oxidation of some carbon-compound and involves the 



