BLOOD VESSELS 457 



be directly acted upon by the condition of the blood and 

 lymph circulating through it. When the blood becomes 

 charged \vith carbon dioxide, as in asphyxia, this centre is 

 stimulated and a general constriction of arterioles with high 

 blood pressure results (p. 548). The same thing happens as 

 a result of a marked decrease in the amount of oxygen in 

 the blood. This leads to an imperfect oxidation of such 

 products as lactic acid, and to their accumulation in the 

 blood, and this concentration of H ions stimulates the vaso- 

 motor mechanism. 



(3) Position of the Centres. — (a) Primary Centre. — In 

 investigating the position of the centre advantage may be 

 taken of — 



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

 at once causes dilatation of the 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 action of 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 with a marked fall in arterial pressure, and prevents 

 the production of reflex constriction by stimulation of an 

 afferent nerve. 



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

 therefore is 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 tectum, 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 stimula- 

 tion stops. 



The centre is therefore one of very considerable longi- 

 tudinal extent. 



(6) Secondary Centres. — It has been found that if, after 



