28 THE FUNCTIONS OF ANIMALS 



ending, and a message travels by a sensory fibre to the 

 nerve-cord. The inner end of the sensory fibre is con- 

 nected with the branches or dendrites of an associative 

 (communicating or internuncial) cell. Thence the message 

 is passed, still within the cord, to the dendrites of a motor 

 nerve-cell. Thence an efferent impulse travels down the 

 axis-cylinder or motor nerve-fibre of the motor neurone to 

 an ending on a muscle fibre, which is thus commanded to 

 contract. The whole nervous system is essentially a con- 

 nected series of such reflex-arcs, all intricately joined up 

 with one another. 



The passage of an impulse along a nerve-fibre may be 

 compared to the passage of a flame along a train of gun- 

 powder. In each case the strength of the initial stimulus 

 does not affect the strength of the transmitted stimulus — 

 the " all-or-none law " is obeyed. In each case the 

 passage of an impulse prevents a second impulse from 

 following close behind it, although the " refractory period " 

 of a nerve-fibre, during which recovery or repair takes 

 place, is extremely short. Oxidation is involved in both 

 cases, though the oxygen consumption of nerves is small. 

 But whereas the train of gunpowder serves to transmit 

 a difference of temperature from one place to another, the 

 nerve-fibre transmits a difference of electric potential from 

 point to point. 



There are two chief kinds of stimuli which are trans- 

 mitted to the central nervous system — stimuli from without 

 the 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, e.g. 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 



