xxv FUNCTIONS OF NERVOUS SYSTEM 289 



would be a very difficult matter in so small an animal as 

 Polygordius. But numerous experiments on a large number 

 of other animals, both higher and lower, allow us to infer 

 with considerable confidence the functions of the various 

 parts in this particular case. 



If a muscle be laid bare or removed from the body in a 

 living animal it may be made to contract by the application 

 of various stimuli, such as a smart tap (mechanical stimulus), a 

 drop of acid or alkali (chemical stimulus), a hot wire (thermal 

 stimulus), or an electric current (electric stimulus). If the 

 motor nerve of the muscle is left intact the application to it of 

 any of these stimuli produces the same effect as its direct 

 application to the muscle, the stimulus being conducted 

 along the eminently irritable but non-contractile nerve. 



Further, if the motor nerve is left in connection with the 

 central nervous system, i.e., with one or more nerve-cells, 

 direct stimulation of these is followed by a contraction, and 

 not only so, but stimulation of a sensory nerve connected 

 with such cells produces a similar result. And finally, 

 stimulation of an ectoderm cell connected, either directly 

 or through the intermediation of a sensory nerve, with the 

 nerve-cells, is also followed by muscular contraction. An 

 action of this kind, in which a stimulus applied to the free 

 sensitive surface of the body is transmitted along a sensory 

 nerve to a nerve-cell or group of such cells and is then, as it 

 were, reflected along a motor nerve to a muscle, is called a 

 reflex action ; the essence of the arrangement is the inter- 

 position of nerve-cells between sensory or afferent nerves 

 connected with sensory cells, and motor or efferent nerves 

 connected with muscles. 



The diagram (Fig. 70) serves to illustrate this matter. 

 The muscle-plate (M. PL) may be made to contract by a 

 stimulus applied (a) to itself directly, (t>) to the motor fibre 



u 



