130 GENERAL FEATURES OF NERVOUS TISSUES. 



modifications of the impulses thus passing through them these ganglia may 

 bring about we do not know. 



It is only in some few instances that we have any indications, and those 

 of a very doubtful character, that the ganglia of the splanchnic system can 

 carry out either of the two great functions belonging to what is physiologi- 

 cally called a nerve centre, namely the function of starting nervous impulses 

 anew from within itself, the function of an automatic centre so called, and the 

 function of being so affected by the advent of afferent impulses as to send 

 forth in response efferent impulses, of converting, as it were, afferent into 

 efferent impulses, the function of a reflex centre so called. 



In the central nervous system, the brain with the spinal cord, which 

 supplies the nervous centres for automatic actions and for reflex actions 

 indeed all the processes taking place in the central nervous system (at 

 least all such as come within the province of physiology) fall into or 

 may be considered as forming part of one or the other of these two 

 categories. 



' 97. 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 away 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, movement of the limbs or body will imme- 

 diately 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 a 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. [Fig. 36.] 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 de- 

 tail, 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 

 Diagram illustrating of the spinal cord in detail hereafter, but it will be 

 Simplest Form of Re- desirable to point out here some of their general fea- 



flex Apparatus.] 



tures. 



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 cer- 

 tain fibres only of a motor nerve, it is only the muscular fibres in which 

 those nerve fibres end which are thrown into contraction. 



