THE VASCULAR MECHANISM. 231 



fall of blood pressure ; this occurs whether the vagus nerves have 

 been previously divided or are intact. In all probability, when the 

 arterial blood pressure is very high, impulses are set up in the endings 

 of the nerve in the stretched aortic wall which reflexly lower the blood 

 pressure, and thus lessen the strain placed upon the heart. The 

 passage of impulses along the depressor nerve in these circumstances 

 can be observed by means of the string galvanometer. 



Increased activity of the centre and a rise of blood pressure are 

 brought about by stimulation of most sensory nerves (see fig. 86), and also 

 by impulses passing to it from the cerebral cortex during muscular 

 exercise and in violent emotional excitement, such as fear or anger. 



(2) The Composition of the Blood. The vaso-motor centre is extremely 

 sensitive to changes in the composition of the blood supplying it, being 

 stimulated by lack of oxygen or by the presence of an excess of carbonic 

 acid in the blood. The effect of lack of oxygen and of excess of carbonic 

 acid is seen in its most extreme form in asphyxia (p. 270), but even 

 a slight excess of carbonic acid stimulates the centre, and leads to 

 constriction of the arterioles and a rise of blood pressure. The same 

 effect is produced when the reaction of the blood, measured by its 

 H ion concentration, becomes less alkaline ; and the injection into the 

 blood stream of small amounts of an acid, such as lactic acid, may 

 produce a considerable rise of blood pressure. Further, the vaso-motor 

 centre is stimulated whenever the amount of blood passing through the 

 brain in a given time diminishes. 



During asphyxia the blood-pressure tracing often shows, in addition 

 to the oscillations caused by the heart beat, two other groups of waves. 

 In the first place, the blood pressure shows oscillations corresponding 

 with the respiratory movements ; they are still present in a curarised 

 animal, and are due to impulses passing by irradiation from the excited 

 respiratory centre to the vaso-motor centre. They are called Traube- 

 Hering curves. Secondly, much larger waves, known as Mayer 

 curves, are seen, and are due to rhythmical variations in the activity 

 of the vaso-motor centre ; they are often present after severe 

 haemorrhage. 



The subsidiary vaso-motor centres, unlike the chief centre, are 

 extremely insensitive to either nervous or chemical stimuli, and probably 

 they take little or no part in the vascular changes normally occurring 

 in the body, though their activity can be excited by asphyxia. 



In whatever way the activity of the vaso-motor centre is increased, 

 the constriction of the blood-vessels which is produced is most pro- 

 nounced in the abdominal organs. The splanchnic nerves send con- 

 strictor fibres to the blood-vessels of almost the whole of the abdominal 



