266 HUMAN PHYSIOLOGY 



Removal of the whole brain above the pons Varolii leaves 

 the centre intact. 



Separation of the pons Varolii and medulla oblongata 

 from the spinal cord at once causes a dilatation of the 

 arterioles of the body, and at once prevents the produc- 

 tion of reflex constriction by stimulation of an afferent 

 nerve. 



The main part, at least, of the vaso-constrictor mechanism 

 is therefore situated in the pons Varolii and medulla 

 oblongata. 



The extent of this centre has been determined by slicing 

 away this part of the brain from above downwards, and 

 studying the influence of reflex stimulation after the removal 

 of each slice. 



It is found that at a short distance below the corpora quad- 

 rigemina, the removal of each succeeding part is followed by 

 a diminution in the reflex constriction, until, at a point close 

 to and just above the calamus scriptorius, all reflex response 

 to stimulation stops. 



The centre is therefore one of very considerable longi- 

 tudinal extent. 



But it has been found that, if, after section of the spinal 

 cord high up, the animal be kept alive for some days, the 

 dilated arterioles again contract, and stimulation of afferent 

 nerves entering the cord below the point of section causes 

 a further constriction. If now another section be made 

 further down the cord, the arterioles supplied by nerves 

 coming from below the point of section will again dilate. 

 This shows that secondary vaso-constrictor centres, tonic 

 in action and capable of having their activity reflexly 

 increased, exist all down the grey matter of the spinal cord. 

 Normally these are under the domain of the dominant 

 centre, but when this is out of action they then come into 

 play. 



B. Yaso-dilator Centre. (a) Mode of Action. This 

 mechanism is not constantly in action. Section of a vaso- 

 dilator nerve does not cause vascular dilatation. It may be 

 excited reflexly, but in a different manner from the vaso- 

 constrictor mechanism. 



Stimulation of an afferent nerve causes a dilatation of the 



