314 



MANUAL OF PHYSIOLOGY. 



through the medulla contains too little oxygen or too much car- 

 bonic anhydride, it stimulates the centre directly and the blood 

 pressure rises. This may be seen by temporarily suspending 

 artificial respiration during an experiment on blood pressure. 

 Reflexly the activity of the vaso-motor centre can be increased 

 by (1) the stimulation of any large sensory nerve, or (2) by sud- 

 den emotion {fear). 



The tone of the arteries may be diminished by lessening the 

 activity of the vaso-motor centre by the stimulation of a peculiar 

 afferent nerve, the anatomy of which has been made out in the 



FIG. 143. 



.OJUUUUUUL AJ J AJ1 . Ji 



Kymograph ic tracing showing the effect on the blood-pressure curve of 

 stimulating the central end of the depressor nerve in the rabbit. The re- 

 cording surface moving from left to right. (c) Commencement and (o) ces- 

 sation of stimulation. There is considerable delay (latency) in both the 

 production and cessation of the effect. (T) Marks the rate at which the 

 recording surface moves, and the line below is the base line. (Foster.) 



rabbit, and probably has its analogue in man, and which passes 

 from the inner surface of the heart to the medulla. The effect 

 of stimulation of this nerve in lowering the pressure is so great 

 that it has been called the depressor nerve. Some emotions (shame) 

 may also reduce the activity of the centre, as is seen in blushing, 

 which is simply dilatation of the facial vessels. 



