VASOMOTOR ACTIONS. 229 



lar fibre is habitually kept on the constrictor side of this equilibrium, and, 

 as in the cases quoted above, may strive of itself toward some amount of 

 active constriction even when separated from the central nervous system. 



But to return to the medullary vasomotor centre. Without attempting 

 to discuss the matter fully, we may say that, after all due weight has been 

 attached to the play of inhibitory impulses or " shock " as a result of opera- 

 tive interference, there still remains a balance of evidence in favor of the 

 view that the region of the medulla of which we are speaking does really 

 act as a general vasomotor centre in the manner previously explained, and 

 plays an important part in the vasomotor regulation of the living body. 



It is not, however, to be regarded as the single vasomotor centre, whence 

 alone can issue tonic-constrictor impulses or whither afferent impulses from 

 all parts of the body must always travel before they can affect the vaso- 

 motor impulses passing along this or that nerve. We are rather to suppose 

 that the spinal cord along its whole length contains, interlaced with the 

 reflex and other mechanisms by which the skeletal muscles are governed, 

 vasomotor centres and mechanisms of varied complexity, the details of 

 whose functions and topography have yet largely to be worked out ; and 

 though, as we have seen, the medullary centre is essentially a centre of im- 

 pulses issuing along vaso-constrictor fibres, it is possible that there are ties 

 between it and vaso-dilator fibres also. As in the absence of the sinus 

 venosus, the auricles and ventricle of the frog's heart may still continue to 

 beat, so in the absence of the medulla oblongata these spinal vasomotor 

 centres provide for the vascular emergencies which arise. As, however, in 

 the normal entire frog's heart, the sinus, so to speak, gives the word and 

 governs the work of the whole organ, so the medullary vasomotor centre 

 rules and co-ordinates the lesser centres of the cord, and through them pre- 

 sides over the chief vascular areas of the body. By means of these vaso- 

 motor central mechanisms, by means of the head centre in the medulla, and 

 the subsidiary centres in the spinal cord, the delicate machinery of the circu- 

 lation which determines the blood-supply, and so the activity of each tissue 

 and organ, is able to respond by narrowing or widening arteries to the ever- 

 varying demands and to meet by compensating changes the shocks and 

 strains of daily life. 



164. We may sum up the history of vasomotor actions somewhat as 

 follows : 



All, or nearly all or, as far as we know, all the arteries of the body are 

 connected with the central nervous system by nerve fibres, called vaso- 

 motor fibres, the action of which varies the amount of contraction of the 

 muscular coats of the arteries, and so leads to changes in calibre. The 

 action of these vasomotor fibres is more manifest and probably more 

 important in the case of small and minute arteries than in the case of 

 larger ones. 



These vasomotor fibres are of two kinds : the one kind, vaso-constrictor 

 fibres, are of such a nature or have such connections at their central origin 

 or peripheral endings that stimulation of them produces narrowing, con- 

 striction of the arteries ; and during life these fibres appear to be the means 

 by which the central nervous system exerts a continued tonic influence on 

 the arteries and maintains an arterial " tone." The other kind, the vaso- 

 dilator fibres, are of such a kind or have such connections that stimulation 

 of them produces widening, dilation of the arteries. There is no adequate 

 evidence that these vaso-dilator fibres serve as channels for tonic dilating 

 impulses or influences. 



The vaso-constrictor fibres leave the spinal cord by the anterior roots of 

 the nerves coming from middle regions only of the spinal cord (in the dog, 



