350 SUMMARY OF VASO-MOTOR ACTIONS. [BOOK i. 



largely to be worked out. 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 spinal bulb 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 

 bulbar 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 bulb, 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 

 j life. 



178. We may sum up the history of vaso-motor actions 

 somewhat as follows. 



In the case of at least a very large number of the arteries of 

 the body we have direct experimental evidence that these arteries 

 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 peripheral endings that stimulation of them produces 

 narrowing, constriction of the arteries. 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;' and this arterial tone may be modified by the 

 action of the central nervous system, so as to give place on the one 

 hand to constriction and on the other to widening. 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 the middle region only of the 

 spinal cord. In the dog, this region extends from about the first 

 or second thoracic to the fourth or fifth lumbar nerve; and in 

 other animals is probably of corresponding extent. Leaving the 

 spinal nerves by the respective visceral branches, rami communi- 

 cantes, the fibres pass into the sympathetic system, the majority 

 joining the main sympathetic chain of ganglia in the thorax and 

 abdomen, but some, for instance those going to certain parts of 



