CH. XXI.] 



VASO-MOTOR CENTRE. 



305 



It can also be excited by the action of poisons in the blood 

 which circulates through it ; thus strophanthus or digitalis causes 

 a marked rise of general arterial pressure due to the constriction of 

 the peripheral vessels brought about by impulses from the centre. 



It is also excited by venous blood, as in asphyxia ; the rise of 

 blood-pressure which occurs during the first part of asphyxia 

 is due to constriction of peripheral vessels ; the fall during 

 the last stage of asphyxia is largely due to heart failure. We 

 shall study asphyxia more at length in connection with 



Fig. 287. Result on arterial blood-prawure curve of stimulating the central end of cut 

 sciatic nerve in rabbit. HP, blood-pressure; A, abscissa or base line; T, time in 

 seconds ; c, signal of period of excitation of the nerve. 



respiration. During the period of decreased pressure, waves are 

 often observed on the blood-pressure curve which arise from 

 a slow rhythmic action of the vaso-motor centre. The centre 

 alternately sends out stronger and weaker constrictor impulses. 

 Tin v are known as the Traube-Hering waves, and are much 

 slower in their rhythm than the waves on the tracing which are 

 due to respiration. They are not peculiar to asphyxia, but 

 are frequently seen in tracings from normal animals. Fig. 286 

 represents tracings obtained from a dog under the influence of 

 morphia and curare. The upper curve taken while artificial 



K.P. x 



