182 KEFLEX ACTIONS. [BOOK i. 



101. Reflex actions. In a reflex action afferent impulses 

 reaching the nervous centre give rise to the discharge of efferent 

 impulses, the discharge following so rapidly and in such a way as to 

 leave no doubt that it is caused by the advent at the centre of the 

 afferent impulses. Thus a frog from which the brain has been 

 removed while the rest of the body has been left intact will 

 frequently remain quite motionless (as far at least as the skeletal 

 muscles are concerned) for an almost indefinite time ; but if its 

 skin be pricked, or if in other ways afferent impulses be generated 

 in afferent fibres by adequate stimulation, movements of the limbs 

 or body will immediately follow. Obviously in this instance the 

 stimulation of afferent fibres has been the cause of the discharge 

 of impulses along efferent fibres. 



The machinery involved in such a reflex act consists of three 

 parts: (1) the afferent fibres, (2) the nerve centre, in this case the 

 spinal cord, and (3) the efferent fibres. If any one of these three 

 parts be missing the reflex act cannot take place ; if for instance 

 the afferent nerves or the efferent nerves be cut across in their 

 course, or if the centre, the spinal cord, be destroyed, the reflex 

 action cannot take place. 



Reflex actions can be carried out by means of the brain, as we 

 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 central portion 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 con- 



