330 YASO-MOTOR CENTRES. [BOOK i. 



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, vaso-motor 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 impulses 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. vaso- 

 motor 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 vaso-motor centre rules and co-ordinates the lesser 

 centres of the cord, and through them presides 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 circulation, which determines the blood supply, and so the 

 activity of each tissue and organ, is able to respond by narrow- 

 ing or widening arteries to the ever-varying demands and to 

 meet by compensating changes the shocks and strains of daily 

 life. 



178. We may sum up the history of vaso-motor 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 vaso-motor fibres is more 

 manifest, and probably more important in the case of small and 

 minute arteries than in the case of large ones. 



These vaso-motor 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, constriction 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, 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 of the spinal cord 

 only (in the dog, and probably in other mammals, from about the 

 second dorsal to the second lumbar nerve), pass into the splanchnic 



