132 GENERAL FEATURES OF NERVOUS TISSUES. 



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 connected with the chest are rhythmically thrown into action and a 

 rhythmically repeated breathing is brought about. And other similar rhyth- 

 mic automatic movements may be carried out by other parts of the spinal 

 cord. 



From the brain itself a much more varied and apparently irregular dis- 

 charge of efferent impulses, not the obvious result of any immediately fore- 

 going 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 

 movements ; 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 of efferent 

 impulses which passing down to the appropriate muscular fibres call forth 

 the rhythmic movement. When we come to study these movements in detail, 

 we shall find reasons for coming to the conclusion that this view is not sup- 

 ported by adequate evidence ; and, indeed, though it is perhaps immature to 

 make a dogmatic statement, all the evidence goes, as we have already said, 

 to show that the great use of the ganglia of the splanchnic system, like that 

 of the spinal ganglia, is connected with the nutrition of the nerves, and that 

 these structures do not, like the central nervous system, act as centres either 

 automatic or reflex. 



99. Inhibitory nerves. We have said that the fibres of the anterior 

 root should be called efferent rather than motor, because though they all 

 carry impulses outward from the central nervous system to the tissues, the 

 impulses which they carry do not in all cases lead to the contraction of mus- 

 cular fibres. Some of these efferent fibres are distributed to glandular struc- 

 tures, for instance to the salivary glands, and impulses passing along these 

 lead to changes in epithelial cells and their surroundings whereby, without 

 any muscular contraction necessarily intervening, secretion is brought about ; 

 the action of these fibres of secretion we shall study in connection with 

 digestion. 



Besides this there are efferent fibres going to muscular tissue or at all 

 events to muscular organs, the impulses passing along which, so far from 

 bringing about muscular contraction, diminish, hinder, or stop movements 

 already in progress. Thus, if when the heart is beating regularly, that is 

 to say, when the muscular fibres which make up the greater part of the 



