282 VETERINARY PHYSIOLOGY 



If any afferent nerve be stimulated, the effect is to increase 

 the activity of the mechanism, to cause a general constriction 

 of arterioles, and thus to raise the general arterial pressure. 

 The centre is. therefore, capable of reflex excitation. In 

 ordinary conditions so many afferent nerves are constantly 

 being stimulated, that it is not easy to say how far the tonic 

 action of this centre is reflex and dependent on the stream of 

 afferent impulses. 



But this centre may also be directly acted upon by the 

 condition of the blood and lymph circulating through it. 

 When the blood is not properly oxygenated and freed from 

 carbon dioxide, as in asphyxia or suffocation, this centre is 

 stimulated and a general constriction of arterioles with high 

 blood pressure results. 



(b) Position. In investigating the position of the centre 

 we may take advantage of 



1st. Its constant tonic influence. Kemoval of the centre 

 at once causes dilatation of arterioles. 



2nd. The fact that it may be reflexly stimulated. If the 

 vaso-constrictor centre be removed, stimulation of an afferent 

 nerve no longer causes constriction of the arterioles. 



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 prevents the production 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 quadri- 

 gemina, 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. 



