202 PRINCIPLES OF PHYSIOLOGY 



this centre is to maintain the arterioles in a 

 certain state of contraction, and this, as already 

 explained, keeps up the blood pressure. But 

 this centre may be inhibited by impulses 

 coming from the heart by the depressor 

 nerve (p. 188), while the centre may be stimu- 

 lated to greater activity by many sensory 

 nerves which thus act as pressor nerves and 

 raise the blood pressure. Again, the centre 

 may be influenced by impressions coming to 

 it from the brain centres, thus producing the 

 blush of shame or the pallor of fear. It has 

 been found that the vaso-motor centre pos- 

 sesses a kind of inherent rhythm. Thus when 

 all peripheral impulses have been prevented as 

 far as possible from reaching it, the blood 

 pressure still goes through a slow series 

 of variations that is to say, the muscular 

 walls of the arterioles have a slow rhythmic 

 movement, contracting and expanding in 

 obedience to impulses still coming from the 

 vaso-motor centre. (Traube-Hering curves of 

 blood pressure). 



115. These nervous mechanisms are pictures 

 of the mode of action of all nerve centres. 



