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



and under special conditions be obtained. Thus in the dog, when 

 the spinal cord is divided in the thoracic region, the arteries of 

 the hind limbs and hinder part of the body, as we have already 

 said, 150, become dilated. This one would naturally expect as 

 the result of their severance from the bulbar vaso-motor centre. 

 But if the animal be kept in good condition for some time, a 

 normal or nearly normal arterial tone is after a while re-estab- 

 lished; and the tone thus regained may, by afferent impulses 

 reaching the cord below the section, be modified in the direction 

 certainly of diminution, i. e. dilation, and possibly, but this is not 

 so certain, of increase, i. e. constriction ; dilation of various cutane- 

 ous vessels of the limbs may be readily produced by stimulation 

 of the central stump of one or another nerve. 



These and other results lead to the conclusion that the bulbar 

 vaso-motor centre is not to be regarded as the sole vaso-motor 

 centre, whence alone can issue tonic constrictor impulses or 

 whither afferent impulses from this or that part of the body must 

 always travel before they can affect the vaso-constrictor 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, vaso-motor centres and mechanisms of varied com- 

 plexity, the details of whose functions and topography have yet 

 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. 



156. 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 connec- 

 tions, that stimulation of them produces widening, dilation of the 



