THE CIRCULATION OF THE BLOOD 697 



briefly here ; for further evidence the reader is referred to my two papers 

 (1908, 2 and 3). 



^ In pressor reflexes we have rise of arterial pressure produced by contraction 

 of peripheral vessels. This is due, mainly, to excitation of the constrictor centre, 

 which sends impulses causing the smooth muscle of the arterioles, already in 

 a state of moderate tonus, to contract still more. Fig. 239 shows that this takes 

 place, because the leg was supplied only by vaso-constrictors, the dilators having 

 been cut. Figs. 240 and 124 (page 416), however, show that constriction can 

 be detected in pressor reflexes when the organ under investigation is supplied only 

 with dilator fibres. In such cases it must be due to the fact that the muscle 

 of the arterioles was kept in a state of inhibition by impulses from the dilator 

 centre, and that the afferent impulses which excite the constrictor centre also 

 inhibit the dilator centre, thus removing the tonic inhibitory influence on the 

 blood vessels, and allowing them to return to their normal state of. tone. It 

 is to be remembered that, as is pointed out by Sherrington, inhibition cannot be 



FIG. 247. EFFECT or STRYCHNINE IN CONVERTING THE DEPRESSOR FALL IN THE 



RABBIT INTO A RISE OF PRESSURE. 



The first stimulation is of the normal nerve. 



A dose of strychnine was given between each stimulation. 



The fall is gradually abolished and its place taken by a rise. 



(Bayliss, 1908, 2, Fig. 15.) 



detected unless there is tonic excitation to be removed. It is, in fact, only in 

 certain states favourable to tonic vascular dilatation, such as high temperature 

 of surroundings and high blood pressure, that it is possible to detect inhibition 

 of dilator tone, apparently on account of the absence of tone in ordinary 

 conditions (see also Fig. 248 below). 



When we turn to depressor reflexes, we find it quite easy to show that the 

 constrictor centre is inhibited. Fig. 120 (page 412) shows this, since the 

 dilators had been cut. Excitation of dilators is shown in the case of the chorda 

 tympani nerve, which contains no constrictor fibres, in Fig. 241. Since, however, 

 this nerve contains secretory fibres, which might be supposed to be excited from 

 afferent fibres in the vagus nerve, although no secretion was observed, objection 

 might be taken to this interpretation. Metabolites might be formed by the cells, 

 and these, as we have seen, cause dilatation of the blood vessels. Fig. 242 is, 

 therefore, more convincing, being the reflex from the depressor nerve of the 

 rabbit, which cannot be held to cause reflex secretion. 



Fofanov and Chalussov (1913) have confirmed these results, and show that 

 vaso-dilatation in the tongue, the nasal mucous membrane, and the legs occurs 

 when the central end of the depressor is stimulated, although the vaso-constrictor 

 supply has been cut off. Fig. 243 is a copy of one of their curves. The case 

 of the leg is interesting, as I had already pointed out (1902, 3, p. 292), on account 



