144 REFLEX ACTIONS. [BOOK i. 



another is that the arborescent terminations of the axis cylinder 

 process of the one cell are applied to the arborescent processes (not 

 of the nature of axis cylinder processes) of the other cell, in such a 

 way that the substances of the two cells are in abundant contact, 

 but not in actual continuity. 



The various nervous centres thus supplied by the grey matter 

 of the spinal cord and brain have two important classes of functions 

 called respectively reflex actions and automatic actions. 



90. 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 csntre, 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 func- 

 tions 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. 



