180 REFLEX ACTIONS. [BOOK i. 



shall see while studying that organ in detail, but the best and 

 clearest examples of reflex action are manifested by the spinal cord ; 

 in fact, reflex action is one of the most important functions of the 

 spinal cord. We shall have to study the various reflex actions of 

 the spinal cord in detail hereafter, but it will be desirable to point 

 out here some of their general features. 



When we stimulate the nerve of a muscle nerve preparation 

 the result, though modified in part by the condition of the muscle 

 and nerve, whether fresh and irritable or exhausted for instance, is 

 directly dependent on the nature and strength of the stimulus. 

 If we use a single induction-shock we get a simple contraction, if 

 the interrupted current we get a tetanus, if we use a weak shock 

 we get a slight contraction, if a strong shock a large contraction 

 and so on; and throughout our study of muscular contractions we 

 assumed that the amount of contraction might be taken as a 

 measure of the magnitude of the nervous impulses generated by 

 the stimulus. And it need hardly be said that when we stimulate 

 certain fibres only of a motor nerve, it is only the muscular fibres 

 in which those nerve fibres end, which are thrown into con- 

 traction. 



In a reflex action on the other hand the movements called forth 

 by the same stimulus may be in one case insignificant, and in 

 another violent and excessive, the result depending on the arrange- 

 ments and condition of the reflex mechanism. Thus the mere 

 contact of a hair with the mucous membrane lining the larynx, a 

 contact which can originate only the very slightest afferent impulses, 

 may call forth a convulsive fit of coughing, in which a very large 

 number of muscles are thrown into violent contractions; whereas the 

 same contact of the hair with other surfaces of the body may pro- 

 duce no obvious effect at all. Similarly, while in the brainless but 

 otherwise normal frog, a slight touch on the skin of the flank will 

 produce nothing but a faint flicker of the underlying muscles, the 

 same touch on the same part of a frog poisoned with strychnia will 

 produce violent lasting tetanic contractions of nearly all the muscles 

 of the body. Motor impulses as we have seen travel along motor 

 nerves without any great expenditure of energy and probably 

 without increasing that expenditure as they proceed ; and the 

 same is apparently the case with afferent impulses passing along 

 afferent nerves. When however in a reflex action afferent impulses 

 reach the nerve centre, a change in the nature and magnitude of 

 the impulses takes place. It is not that in the nerve centre the 

 afferent impulses are simply turned aside or reflected into efferent 

 impulses ; and hence the name "reflex" action is a bad one. It is 

 rather that the afferent impulses act afresh as it were as a stimulus 

 to the nerve centre, producing according to circumstances and con- 

 ditions either a few weak efferent impulses or a multitude of strong 

 ones. The nerve centre may be regarded as a collection of explo- 

 sive charges ready to be discharged and so to start efferent 



