178 General Biology 



an organism which cause protective reactions when harmful stimuli are 

 applied. For example, a frog, even after both cerebral hemispheres are 

 removed, will still scratch the part of its body to which a drop of acid 

 is applied, and it will even snap at and swallow a fly which has been 

 placed on the tip of its nose. Again, a fly will walk, fly, and clean its 

 legs and wings, after its head is entirely removed, and the writer has 

 kept a decapitated cat alive for many hours by artificial respiration and 

 caused it to perform many instinctive actions such as scratching itself, 

 waving its tail, etc. 



In order to understand Reflex Action, 

 it is first necessary to know the meaning of 

 a Nerve Arc (Fig. 84). This latter is 

 merely the entire nerve-path over which an 

 impulse passes to a nerve-center and out 

 again to a muscle cell. It must, therefore, 

 consist of the nerve-ending of a sensory 

 Fig. 84 Diagram of the path of a nerve (receptor) through which the impulse 



simple nervous reflex action. . . 



is received, and the sensory nerve-fiber 

 which carries the impulse to the nerve-center to join at this point with 

 a motor-nerve fiber which in turn carries the motor impulse to the 

 motor-nerve-ending (affector). This motor-nerve-ending is always 

 located in some muscle fiber. Psychology textbooks often speak of a 

 nerve-arc, as "a perception with a motor impulse." 



A Simple Reflex Action is one that passes over such a simple nerve- 

 arc without first passing to the higher nerve-centers, or, we may say, 

 one which does not come into the consciousness of the individual in 

 whom the action takes place. Such a reflex action is, therefore, purely 

 physical. There is no need of assuming any mental state or sensation 

 as an accompaniment. 



When an individual is born, his nerve arcs are set in some form 

 or another, so that with one individual the same stimulus will cause 

 quite a different reaction, than it will in another. But, just because 

 fhese nerve-arcs are set in the way they are, the same nerve-arc will 

 always react in the same way to the same stimulus, if all other condi- 

 tions are equal. For example, a child may have grown accustomed to 

 saying "I is" for "I am" and have said it so often that it finds it very 

 difficult to correct itself. Now, if we constantly force the child to use 

 the form "I am," the particular nerve-arc which carried the "I am" 

 reaction will become relatively stronger than the one which carried the 

 reaction "I is," and then, and not until then, does the latter phrase 

 become a sort of second nature to the child. 



So, too, a puppy that has the vicious habit of snapping at passers-by, 

 can be made to react differently by giving him a whipping several times, 

 immediately after he does the undesirable act. 



