184 INHIBITORY NERVES. [BOOK i. 



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 

 which make up the greater part of the heart are rhythmically 

 contracting, the branches of the pneumogastric nerve going to the 

 heart be adequately stimulated, for instance with the interrupted 

 current, the heart will stop beating ; and that not because the 

 muscles of the heart are thrown into a continued tetanus, the 

 rhythmic alternation of contraction and relaxation being replaced 

 by sustained contraction, but because contraction disappears alto- 

 gether, all the muscular fibres of the heart remaining for a 

 considerable time in complete relaxation and the whole heart 

 being quite flaccid. If a weaker stimulus be employed the beat 

 may not be actually stopped but slowed or weakened. And, as we 

 shall see, there are many other cases where the stimulation of 

 efferent fibres hinders, weakens, or altogether stops a movement 

 already in progress. Such an effect is called an inhibition, and 

 the fibres stimulation of which produces the effect are called 

 'inhibitory' fibres. 



The phenomena of inhibition are not however confined to 

 such cases as the heart, where the efferent nerves are connected 

 with muscular tissues. Thus the activity of a secreting gland may 

 be inhibited, as for instance when emotion stops the secretion of 

 saliva, and the mouth becomes dry from fear. In this instance 



