4 i8 



PRINCIPLES OF GENERAL PHYSIOLOGY 



neighbourhood of the cathode, a propagated disturbance can be set up by a smaller 

 stimulus than in the normal state, and in the neighbourhood of the anode a 

 stronger stimulus is required. The excitability is increased at the cathode ; 

 diminished at the anode. 



Chemical Ayents. We see then that inhibition, as well as excitation, can be 

 brought about by means other than stimulation of nerves. 



Various chemical agents, as we saw in discussing the action of electrolytes in 

 general, can produce a state of inexcitability. The hypothesis of Howell, as to the 

 functipn of potassium in the action of the vagus nerve, was also referred to in that 



connection. 



It is interesting to 

 note that excitation by 

 rlicinir.-il nit-fins can be 

 antagonised by nervous 

 inhibition, as shown in 

 Fig. 125, which shows 

 that the excitation of 

 the vaso-constri<-t Hi- 

 centre by asphyxi;il 

 blood is neutralised by 

 simultaneous stimulation 

 of the depressor nerve 

 (see Bayliss, 1893, p. 

 319). 



Fig. 126 is another 

 interesting case. The 

 tracing is that of the 

 respiratory movements 

 of the slip of the dia- 

 phragm in the rabbit 

 which is attached to tin- 

 end of the sternum, as 

 used by Head for the 

 investigation of respi- 

 ratory reflexes. At the 

 commencement of the 

 figure it is completely 

 relaxed and at rest, the 

 vagus nerves being 

 intact. The series of 

 contractions was caused 

 by the administration of 

 air containing carbon 

 dioxide. At x the vagus 

 nerves were cut; the 



muscle is seen to pass into a state of partial tonic contraction, owing to excitation 

 from the respiratory centre, which had previously been inhibited by impulses from 

 the vagus endings in the lungs. A second inhalation of carbon dioxide excites 

 respiratory movements of the slip, and the point of interest is that its tonus is 

 inhibited, so that in expiration th'e position is that of nearly complete relaxation. 



FIG. 125. ANTAGONISM OF DEPRESSOR INHIBITION AND 

 ASPHYXIAt, STIMULATION OF THE VASO-CONSTRICTOR CENTRE. 

 Curarised rabbit. Arterial pressure curve. 



Depressor stimulated from A to D. 



The pressure would have remained throughout at the level of B if the 

 artificial respiration had not been stopped at this point. The result 

 of the asphyxial excitation is to bring back the arterial pressure to 

 normal. Respiration was resumed at C, and, as the asphyxial state 

 disappears, the depressor fall again shows itself until the final cessation 

 of the stimulus to the nerve. 



(Bayliss, 1893, Fig. 24.) 



THEORIES OF INHIBITION 



The facts brought out in the several cases described above will, as it seems to 

 me,- serve to show that it is not" to be supposed that the inhibition of a process is 

 in all cases effected in the same way. A general theory of inhibition is thus 

 improbable, although there is no doubt that all the cases have the same essential 

 basis and, when we consider that the process of inhibition is the opposite of that 

 pf excitation, we realise that when we know more about the latter, we shall know 



