DEPRESSOR NERVE. 319 



studied. The small arteries dilate and the pulse becomes appar- 

 ent in the capillaries, and even in the veins. From these facts it 

 seems highly probable that in the medulla oblongata a vasomotor 

 centre exists, which regulates the contraction of all the vessels, 

 and keeps them constantly more or less contracted; the centre 

 receiving some continuous stimulation, which results in a slight 

 general vascular constriction or arterial tone. The existence of 

 such a centre in the medulla, and of nerve channels in the cord 

 leading from such a centre, is made certain by the fact, that if a 

 gentle stimulus be applied to a certain part of the medulla, or 

 just below it, simultaneous general vascular constriction sets in, 

 and is indicated by a great and sudden rise in the blood pressure. 



The action of the vasomotor centre can be increased, and 

 thereby the tone of the vessels elevated, and the pressure raised, 

 either by (1) direct or (2) reflex excitation. If the blood flowing 

 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 vasomotor 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 vasomotor centre by the stimulation of a peculiar 

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

 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. 



Besides this chief vasomotor centre, it is probable that in the 

 higher animals, as certainly is the case in the frog, other centres 

 are distributed throughout the spinal cord, which seem to be able 

 to take the place of the great primary centre. For after the 

 spinal cord has been cut high up, the hinder extremities more or 

 less recover their vasomotor power in a few days, and destruction 



