292 CIRCULATION OF THE BLOOD. 



kymograph, and the vagus of the same side divided opposite 

 the thyroid cartilage. The depressor is isolated, and a loop 

 of thread passed round it. An observation is then taken of 

 the arterial pressure and pulse rate, after which the depressor 

 is divided. There is no alteration either in the height of the 

 mercurial column, or in the number of pulsations per ten 

 seconds. On exciting the peripheral end, there is still no 

 effect; but on exciting the central end the pressure sinks to 

 about two-thirds of its previous height, and the pulse often be- 

 comes slower. On discontinuing the excitation, the status quo* 

 is gradually restored. 



The results of such an experiment are shown in the tracing 

 (fig. 245). It is seen that the excitation produces no change 

 whatever either in the character or frequency of the pulsations, 

 the only effect produced being diminution of pressure. In other 

 instances there is perceptible slowing, but the variations of the 

 two effects are never parallel. In the observation recorded in 

 the tracing, the vagus of the side opposite to that on which the 

 depressor was excited, was left intact; consequently the heart 

 was still partly under the control of the intracranial inhibitory 

 centre. Notwithstanding this, the slowing was not appreci- 

 able. When it does occur, it must be attributed, without doubt, 

 to reflex excitation of the inhibitory heart centre, the effect of 

 which is conveyed to the heart by the undivided vagus. 



The diminution of the arterial pressure cannot be referred 

 to any direct influence exercised by excitation of the depressor 

 on the heart, but to diminution of the resistance in the arterial 

 system ; i. e., to relaxation of the minute arteries. This may 

 be shown in the same animal which is used for the preceding 

 experiment, if the left splanchnic is divided (see 56) and the 

 depressor excited as before. The mercurial column, which has 

 alread}' fallen, say, to two-thirds of its former height, is further 

 depressed during excitation ; but the amount of sinking is 

 much less than it would have been if the splanchnic had not 

 been divided. 



The same conclusion is confirmed by two other observations, 

 viz., (1) that if the aorta is obstructed so as to raise the arterial 

 pressure and conceal any changes in the state of contraction 

 of the abdominal vessels, the effect of the excitation of the de- 

 pressor is imperceptible : and (2) that if the abdominal organs 

 are exposed and inspected during excitation of the depressor, 

 they are seen, according to Cyon, to become congested. The 

 effect is most perceptible in the kidneys, which (if care is taken, 

 to avoid the previous occurrence of congestion from expos'ure 

 or other conditions) change color from pale to red, and back 

 again, as the induced current is closed or opened. 



