CHAP, in.] GENERAL FEATURES OF NERVOUS TISSUES. 187 



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, 

 however, it is probable that inhibition is brought about not by 

 inhibitory impulses passing to the gland and arresting secretion 

 in the gland itself, but rather by an arrest, in the central nervous 

 system, of the nervous impulses which, normally, passing down to 

 the gland, excite it as we shall see to action. And indeed, as we 

 shall see later on, there are many illustrations of the fact that 

 afferent impulses reaching a nervous centre, instead of stimulating 

 it to activity, may stop or inhibit an activity previously going on. 

 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, hinder, or inhibit 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. It is sufficient for us 

 at the present to recognize that a nervous impulse passing along 

 a nerve fibre need not always set free energy when it reaches 

 its goal, it may hinder or stop the setting free of energy and is 

 then called an inhibitory impulse. 



