186 INHIBITORY NERVE. [BOOK i. 



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 

 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 supported 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 shew 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. 



103. 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 muscular fibres. Some of 

 these efferent fibres are distributed to glandular structures, 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 



