CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 185 



highly complex; and that it is the constitution and condition of 

 the centre which determines the complexity and character of the 

 movements which are effected. In other words, a centre concerned 

 in a reflex action is to be regarded as constituting a sort of 

 molecular machinery, the character of the resulting movements 

 being determined by the nature of the machinery set going and 

 its condition at the time being, the character and amount oFtrle 

 afferent impulses determining exactly what parts of and how far 

 the central machinery is thrown into action. 



Throughout the above we have purposely used the word 

 centre, avoiding the mention of nerve cells. But undoubtedly the 

 part of the spinal cord acting as centres of reflex action is situated 

 in the grey matter, which grey matter is characterised by the 

 presence of nerve cells; undoubtedly also the efferent fibres are 

 connected with the afferent fibres by means of cells, certainly by 

 the cells of the anterior cornu described in 99 and probably also 

 by other cells in the posterior cornu or elsewhere. So that a 

 reflex action is carried on undoubtedly through cells. But it does 

 not follow that a cellular mechanism is essential, in the sense at all 

 events that the nuclei of the cells have anything to do with the 

 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 impulse's 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 central nervous system called the spinal 

 bulb, or medulla oblongata, changes of the nervous material, re- 

 curring rhythmically, lead to the rhythmic discharge along certain 

 nerves of efferent impulses whereby muscles connected 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 various 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 



