CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 179 



function the multipolar cells of the splanchnic ganglia appear to 

 serve the purpose of multiplying the tracts along which nervous 

 impulses may pass. An impulse for instance reaching a multipolar 

 cell in one of the proximal (sympathetic) ganglia along one 

 fibre or process (the fibre in very many cases being a medullated 

 fibre) can pass out of the cell in various directions along several 

 processes or fibres, which in the majority of cases if not always are 

 non-medullated fibres. Thus these nerve cells are organs of dis- 

 tribution for impulses of the same kind. What further modifica- 

 tions 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 physiologically 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. 



It is 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. 



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 bv the advent at the centre of the 



t/ 



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 



122 



