CHAP. HI.] GENERAL FEATURES OF NERVOUS TISSUES. 183 



matter, or even that the most important of the molecular processes 

 constituting the changes taking place in a centre during a reflex 

 action are carried out only by the cell substance immediately 

 surrounding the nuclei. The power of carrying out a reflex action 

 is probably contingent on the nature and arrangement of axis- 

 cylinders, and of the branching material by which in a nerve 

 centre the afferent and efferent axis-cylinders are joined together, 

 the nuclei intervening only so far as they have to do with the 

 growth and repair of the nervous material. 



102. Automatic actions. Efferent impulses frequently issue 

 from the brain and spinal cord and so give rise to movements 

 without being obviously preceded by any stimulation. Such move- 

 ments are spoken of as automatic or spontaneous. The efferent 

 impulses in such cases are started by changes in the nerve centre 

 which are not the immediate result of the arrival at the nerve 

 centre of afferent impulses from without, but which appear to 

 arise in the nerve centre itself. Changes of this kind may recur 

 rhythmically ; thus, as we shall see, we have reason to think that 

 in a certain part of the medulla oblongata changes of the nervous 

 material, recurring rhythmically, lead to the rhythmic discharge 

 along certain nerves of efferent impulses whereby muscles con- 

 nected with the chest are rhythmically thrown into action and a 

 rhythmically repeated breathing is brought about. And other 

 similar rhythmic automatic movements may be carried out by 

 other parts of the spinal cord. 



From the brain itself a much more varied and apparently 

 irregular discharge of efferent impulses, not the obvious result of 

 any immediately foregoing afferent impulses, and therefore not 

 forming part of reflex actions, is very common, constituting what 

 we speak of as volition, efferent impulses thus arising being called 

 volitional or voluntary impulses. The spinal cord apart from the 

 brain does not appear capable of executing these voluntary move- 

 ments ; but to this subject we shall return when we come to speak 

 of the central nervous system in detail. 



We said just now that there is no satisfactory evidence 

 that the ganglia of the splanchnic system ever act as centres of 

 reflex action. The evidence however that these ganglia may 

 serve as centres of rhythmic automatic action seems at first sight 

 of some strength. Several organs of the body containing muscular 

 tissue, the most notable being the heart, are during life engaged 

 in rhythmic automatic movements, and in many cases continue 

 these movements after removal from the body. In nearly all 

 these cases ganglia are present in connection with the muscular 

 tissue ; and the presence and intact condition of these ganglia 

 seem at all events in many cases in some way essential to the due 

 performance of the rhythmic automatic movements. Indeed it 

 has been thought that the movements in question are really due 

 to the rhythmic automatic generation in the cells of these ganglia 



