Irritation of a sensory nerve usually produces reflex stimulation of the vaso-motor centre and consequent contraction of the vessels and rise in the blood-pressure both in the frog and higher animals. The chief vaso-motor centre is situated in the medulla oblongata, but it is probable that there are many subsidiary centres throughout the body. It is probable also that these vary in strength and in the amount of independent action they possess in different animals. When the influence of the chief vaso-motor centre upon the body is destroyed by section of the spinal cord just below the medulla, the vessels dilate and the blood-pressure falls greatly. This is, however, not always the case, for in some dogs I have noticed that after section of the medulla, the blood-pressure remained so high that I was under the impression that the cord had been imperfectly divided, yet after death examination of the cord showed that section was complete.

The vaso-motor centre is paralysed by numerous drugs, especially in the final stages of their action, so that its ordinary tonic action is destroyed and the blood-pressure falls greatly. Its action of responding to a reflex stimulation is also abolished, and irritation of a sensory nerve no longer raises the pressure. The tonic and reflex action of the centre do not always appear to be effected pari passu, - chloral, for example, appearing to have a greater power to diminish its reflex action than its tone, so that stimulation of a sensory nerve has little or no effect even when the blood-pressure has not as yet fallen very low. Sometimes, indeed, an opposite effect to the usual one may be produced and the blood-pressure be lowered still further instead of raised by the stimulation. Alcohol also paralyses very markedly both the reflex power and the direct excitability of the vasomotor centre, so that neither stimulation of a sensory nerve, nor even stimulation of the centre of suffocation, will raise the blood-pressure.1 Both the normal tone and the reflex excitability of the vaso-motor centre are greatly increased by strychnine. The general blood-pressure greatly rises after the injection of this drug, and the effect of irritation of a sensory nerve upon it is increased. It has already been mentioned that in ordinary circumstances the subsidiary vaso-motor centres in the cord when separated from the medulla cannot of themselves maintain the blood-pressure. After the injection of strychnine, however, their action is so much increased that they may keep the blood-pressure at a high average and may also cause it to rise on irritation of a sensory nerve.