146 AUTOMATIC ACTIONS. [BOOK i. 



motor nerve which is the fellow of the sensory nerve, the stimula- 

 tion of which calls forth the movement. In the more complex 

 reflex actions of the brainless frog, and in other cases, the relation 

 is of such a kind that the resulting movement bears an adaptation 

 to the stimulus : the foot is withdrawn from the stimulus, or 

 the movement is calculated to push or wipe away the stimulus. 

 In other words, a certain purpose is evident in the reflex action. 



Thus in all cases, except perhaps the very simplest, the move- 

 ments called forth by a reflex action are exceedingly complex 

 compared with those which result from the direct stimulation of a 

 motor trunk. 



91. 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 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 

 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. 



While reflex and automatic actions are thus frequently carried 

 out by the grey matter of the central nervous system, of which grey 

 matter nerve cells are conspicuous constituents, it is at least not 

 absolutely proved that either kind of action is carried out by the 

 other portions of the nervous system in which nerve cells are 

 found. 



As regards the ganglia on the posterior roots of spinal 

 nerves it can be definitely affirmed that these act neither as auto- 

 matic centres nor as centres of reflex action. The nerve cell of 

 such a ganglion serves to govern the nutrition of the afferent 

 nerve fibre to which it is attached by a T shaped junction ; and a 

 portion of the fibre which is cut away from its nerve cell in the 



