CHAP, iv.] THE VASCULAR MECHANISM. 211 



In the third place, the cases of the corpora cavernosa of the 

 penis and the submaxillary gland suggest the idea that dilation is 

 the result of the complete or partial loss of local tone, that in fact 

 vaso-dilators act by inhibiting, and vaso-constrictors by augmenting, 

 the activity of the local mechanism (whatever it be) which gives rise 

 to the local tone. The erection of the penis which follows stimula- 

 tion of the nervi erigentes, and the injection of the submaxillary 

 gland which follows stimulation of the chorda tympani, present a very 

 close analogy to the inhibition of the heart by stimulation of the 

 vagus. Just as the rhythmic contraction of the cardiac fibre is 

 stopped by the vagus, so the tonic contraction of the arterial fibre 

 (and this tonic contraction is indeed at bottom an obscure rhythmic 

 contraction) is stopped by the chorda or the nervi erigentes. And 

 it seems to be very natural to draw the conclusion that dilation is in 

 all cases mere inhibition, and constriction in all cases mere augmen- 

 tation, of local tone. But tempting as this view is, and useful perhaps 

 as it may be as a working hypothesis, it must not be regarded as 

 definitely proved. It is quite possible that dilation may be brought 

 about in different ways in different cases ; and so also with con- 

 striction. 



Further, the occurrence of dilation after simple section of a 

 nerve raises an interesting question. Do the arteries in such a case 

 dilate because the very section of the nerve acts as a stimulus to 

 vaso-dilator fibres, or because the local tone is insufficient to keep 

 up an adequate arterial constriction unless it be supplemented by 

 additional tonic impulses reaching the local mechanism from the 

 central nervous system, which supplement is lost by section of the 

 nerve ? Obviously, if mere section behaves as a stimulus to vaso- 

 dilator fibres of such a potency as to give rise to a dilation lasting 

 hours or it may be days, all evidence of 'tonic' impulses proceeding 

 from the central nervous system is done away with. We can then 

 only speak of dilation and constriction as being the result of the action 

 of vaso-dilator and vaso-constrictor fibres respectively, both worked 

 in a reflex manner by the central nervous system. Into the dis- 

 cussion whether such an interpretation of the effects of simple 

 section is justified by facts or not, and into the allied controversy 

 concerning the reason why the vaso-motor effects of stimulating the 

 efferent fibres of the sciatic and other nerves vary so much under 

 different circumstances, we cannot enter here. We must content 

 ourselves with the general conclusion that though local tone may 

 exist independently of the central nervous system, the condition of 

 the various vascular areas, in the living body in a normal condition, 

 is arranged and modified to meet passing or permanent needs, by 

 the central nervous system, through the agency of vaso-motor nerves, 

 and that these vaso-motor nerves in some cases, since they are used 

 to give rise to dilation only, may be spoken of as vaso-dilator nerves, 

 or as containing vaso-dilator fibres, in other cases may similarly be 

 called vaso-constrictor, and in yet a third class of cases be regarded 



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