REFLEX ACTION 47 



the conscious effort is rather to prevent the reflex from 

 taking place, as is apt to be true when we sneeze. Of 

 some reflexes we are quite unlikely to be aware; this is the 

 case with the narrowing of the pupil in response to in- 

 crease of light. 



Let us now go into some detail and analyze carefully 

 the reflex process. We have seen that the primary cause 

 is an external change of some sort, the word " external " 

 meaning outside the central nervous system and not 

 necessarily outside the body. The change which is at the 

 root of the reflex is usually referred to as an external 

 stimulus. It would be easy to give a long list of exam- 

 ples. A foreign particle comes in contact with the larynx; 

 its contact is the stimulus which develops the coughing 

 reflex. Slight drying of the exposed surface of the eyeball 

 is a common cause of the winking reflex. Irritation of the 

 lining of the stomach is the most frequent of the many 

 possible stimuli through which vomiting can be excited. 



External stimuli would fail of any extended effect if it 

 were not for the nervous connections of the parts affected. 

 In the last chapter the nervous system was spoken of as 

 sending its impulses out to muscles and glands. But 

 its work is twofold. It not only acts, but it is acted upon. 

 Its fibers fall into two classes, those which are concerned 

 with transmission of effects outward from the brain and the 

 spinal cord, and those having the opposite function, the 

 carrying inward of impulses started by external causes. 

 The first class of conductors are usually called motor; the 

 second, sensory. Both terms are open to objection, as a 

 little consideration will show. The effects which the ner- 

 vous system produces in the tissues of the body are not 

 solely movements. The word " motor," then, is not in- 

 clusive enough. It is better to substitute the word 

 efferent, 1 which means simply centrifugal, and which im- 

 plies nothing whatever about the nature of the responses 

 evoked. Efferent fibers may be motor, that is, exciting 

 contraction, but they may also inhibit contraction, and 

 1 Efferre, to bear away. 



