148 AUTOMATIC ACTIONS. [BOOK i. 



the rhythmic movement. When we come to study these move- 

 ments in detail, we shall find reasons for coming to the conclusion 

 that this view is not supported by adequate evidence. 



92. 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, secre- 

 tion 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. In fact it is probable, though not actually 

 proved in every case, that wherever in any tissue, energy is being 

 set free, nervous impulses brought to bear on the tissue may affect 

 the rate or amount of the energy set free in two different ways ; on 

 the one hand, they may increase or quicken the setting free of 

 energy, and on the other hand they may slacken or hinder the 

 setting free of energy. And in at all events a large number of 

 cases it is possible to produce the one effect by means of one set 

 of nerve fibres, and the other effect by another set of nerve fibres. 

 We shall have occasion however to study the several instances of 

 this double action in the appropriate places. 



