EXCITATION AND INHIBITION 387 



relation to the secreting glands may be mentioned here. If the varying degrees of 

 secretory activity, to be obtained by gradation of the stimulus to the nerve, be due 

 to maximal stimulation of a greater or less number of fibres, evidence should be 

 obtained in microscopic appearances that some cells or alveoli are much more 

 fatigued than others. If the figures on pp. 957 and 982 of Metzner's article 

 (1907, 2) be referred to, it will be seen that this is actually the case. 



A question cognate to the last, and of considerable importance, is whether 

 electrical or other stimuli of different time course are able to produce nerve 

 disturbances of different kinds. This would appear from Adrian's results to be 

 improbable, but it has been found that the comparatively slowly rising current, to 

 be obtained from a rheonome, caused an abnormally long twitch of the muscle to 

 which the nerve was attached. Subsequent investigation with an instrument able 

 to detect the existence of disturbances following one another at very brief intervals, 

 showed that several successive impulses passed down the nerve in such cases. It 

 appears that different nerve fibres are excited by the slowly rising current at 

 different times after it begins to pass. Dr Keith Lucas, to whom I owe this 

 information, also states that he is unaware of any evidence to show that the nerve 

 impulse is in any way modified by the nature of the stimulus. The different forms 

 of electrical change in nerve must, therefore, be ascribed to a series of impulses, if 

 it be supposed that they represent the process in a single fibre ; but it seems more 

 likely that a varying number of fibres are being excited in rotation. A case of 

 this kind is shown in Fig. 106, D and E (Einthoven), where the electrical change 

 in the vagus nerve, produced by inflation of the lungs, is seen to follow precisely 

 the degree of inflation, and might be explained on the hypothesis that the receptive 

 end organs in the lung tissue are of varying degrees of sensibility, so that all would 

 be excited by a strong inflation, but fewer and fewer in proportion as the degree of 

 stretching decreases. 



That the result produced by the impulses travelling in a nerve depends on the 

 way the fibres end, arid not on any difference in the impulses themselves, is 

 shown by Langley's experiments (1898) on the union of different nerves. When 

 the central end of the vagus is joined to the peripheral end of the cervical 

 sympathetic, and regeneration has taken place, stimulation of the vagus produces 

 the same effects as that of the cervical sympathetic did previously. Moreover, 

 reflexes produced by afferent impulses, which excite efferent fibres of the vagus 

 in the normal state, instead of producing cardiac inhibition, cause contraction of 

 the arterioles of the ear, together with the other effects of stimulation of the 

 sympathetic. The central end of the lingual was joined to the peripheral end 



B, Depressor nerve. Rabbit. 



First curve Electrical change in the heart end of the cut depressor nerve. 



Second curve Respiratory movements. 



Third curve Heart beats. 



1 mm. abscissae = '2 second 1 mm. ordinates = 5 microvolts. 



Note the electrical change with each heart beat, none with the respirations. 



C, Vagus nerve. Dog. 



First curve Electrical changes in the thoracic end of the cut vagus. 



Second curve Respiratory movements. 



Third curve Blood pressure with heart beats. 



Fourth line Stimulation signal. 



1 mm. abscissae = 0"2 second. 1 mm. ordinate = 6'7 microvolts. 



Note that both heart and lung produce electrical effects in the nerve, since the vagus trunk contains the 



depressor fibres. 

 At the rise of the signal the peripheral end of the vagus of the opposite side was stimulated. Respiration 



continues, with its electrical effect. The heart beat stops and, with it, the depressor waves cease. 



D and E, Vagus. Dog under artificial respiration. 

 In D air is rhythmically blown into the lungs. 

 In E, after a pause, air is sucked out four times, commencing at a. 

 First curves Electrical change in thoracic end of vagus. 

 Second curves Movements of chest upwards means inflation. 

 Third curves Blood pressure with heart beats. 

 Fourth line Signal. 



1 mm. abscissae = 0'2 second. 1 mm. ordinates = 9 microvolts. 

 Note how the electrical change coincides with the curves of distension and continues during the whole 



the same as that with distension, but of less magnitude, 

 seen, owing to the decreased sensibility of the galvano- 

 posite vagus was stimulated. 



(From curves kindly sent by Prof. Einthoven.) 



