EXCITATION AND INHIBITION 



403 



It is unnecessary to state that an actual destructive process is not spoken of as an 

 inhibition. The possibility 

 of return to the normal state, 

 capable of again entering into 

 a condition of excitation, is 

 essential. In some cases, 

 however, it may happen that 

 a vicious circle is established, 

 so that the normal state does 

 not return, as we shall see 

 later. 



As a general state- 

 ment it may be taken 

 that the processes at the 

 basis of inhibition are of 

 an opposite kind to those 

 at the basis of excitation, 

 so that the explanation of 

 the one is involved in 

 that of the other. If 

 there is an increase of 

 permeability in excita- 

 tion, a decrease is to be 

 expected in inhibition, for 

 example. It will be 

 found, however, that the 

 definite knowledge we 

 possess of the nature of 

 the inhibitory process is 

 still more defective than 

 that of the excitatory 

 process, so that all facts 

 bearing on the question 

 are of value. 



FIG. 110. EFFECT ON THE VESSELS OF THE TONGUE OF THE 

 DOG OF STIMULATION OF THE VASO-CONSTKICTOR AND OF 

 THE VASO-DILATOK NERVES. 



Upper curve volume of the tongue, obtained by a plethysmograph, so 

 that a fall means decrease of volume, due to constriction of arterioles 

 allowing a smaller amount of blood to be present ; and vice versa 

 when the curve rises. 



Lower curve general arterial pressure, showing that the changes in the 

 tongue are not due to changes in the pressure at which the blood is 

 supplied to it. 



Upper of the two lines signal, marking at the first rise, stimulation of the 

 peripheral end of the cervical sympathetic nerve, which supplies vaso- 

 constrictor fibres ; at its second rise, that of the peripheral end of the 

 lingual nerve, which contains the vaso-dilator fibres. 



Bottom line time in ten-second intervals. 



To obtain a general 



view of the kind of facts to be explained, some experimental cases will be 



useful. They will also serve to 

 indicate the different kinds of in- 

 hibition to which reference was 

 made above. 



Smooth Muscle. We have 

 already seen how the intestine shows 

 a series of rhythmic contractions, 

 superposed on a background of 

 moderate tonic contraction, and how 

 these contractions and tone can be 

 reduced by the splanchnic nerves 

 and increased by the vagus nerves 

 (see Figs. 96 and 97, page 369). 

 Since the excitation of the spon- 

 taneous contractions proceeds from 

 the muscle itself, the action of the 

 inhibiting and exciting nerves is 

 exercised on the muscle cells directly. 

 In other cases, as the muscular 

 coat of the arterioles, there is no 

 evidence that the plexus of nerve 

 fibres supplying the contractile cells 

 contains nerve cells (see Fig. 109), 

 so that the state of tonic contraction 



is clearly inherent in the muscle itself. It can be increased by nerves called vaso- 



FIG. 111. TRACING FROM RETRACTOR PENIS 

 MUSCLE OF THE DOG. 



The fall in the curve is produced by lengthening of the 

 muscle, brought about by stimulation of the pelvic 

 nerve, which inhibits the spontaneous tonic state. 



The rise is contraction, due to stimulation of the pudic nerve, 

 which increases the degree of the tonic contraction. 



(Bayliss and Starling.) 



