The nerve cells which govern the majority of the vasomotor channels, lie in the upper part of the medulla oblongata in the floor of the fourth ventricle. This is proved by two facts: 1st, most of the brain may be removed without diminishing the arterial tone; and 2d, if the spinal cord be cut below the medulla (artificial respiration of course being kept up), the mean blood pressure is found to fall immediately, almost to the level of the atmospheric pressure, owing to the relaxation of the smaller arteries consequent on the paralysis of their muscular coat.

The changes in the capillary circulation caused by vascular paralysis can be seen in the web of a frog in which the medulla has been destroyed (pithed) while the circulation is being studied. The small arteries dilate and the pulse becomes apparent in the capillaries, and even in the veins.

It seems probable that in the medulla oblongata a vasomotor centre exists, which can regulate the contraction of all the vessels, and keep them constantly more or less contracted. This slight general vascular constriction is spoken of as the arterial tone. The existence of such a centre in the medulla, and of nerve channels in the cord leading from it, 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, as indicated by a great and sudden rise in the blood pressure.

Kymographic tracing showing the effect on the blood pressure curve of stimulating the central end of the depressor nerve in the rabbit.

Fig. 143. Kymographic tracing showing the effect on the blood-pressure curve of stimulating the central end of the depressor nerve in the rabbit. The recording surface moving from left to right. (C) Commencement and (O) cessation 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).

Pressor Influences

The action of the vasomotor centre can be increased, the tone of the vessels elevated, and the pressure raised, either by (i) direct or (2) reflex excitation. Directly, if the blood flowing through the medulla contains too little oxygen or too much waste products it stimulates the centre and the blood pressure rises. This may be seen by temporarily suspending artificial respiration during an experiment on blood pressure, when the pressure rises considerably. Reflexly, the activity of the vasomotor centre can be increased by (i) the stimulation of any large sensory nerve or (2) by sudden emotion (/ear).

Depressor Influences

The tone of the arteries may be diminished by inhibiting the activity of the vasomotor centre by the stimulation of a certain afferent nerve, the anatomy of which has been made out in the rabbit and some other animals, and probably has its analogue in man. It passes from the inner surface of the heart to the vasomotor centre in the medulla. The effect of stimulation of this nerve in lowering the blood pressure is so great that it is called the depressor nerve. Some emotions {shame) may also reduce the activity of the centre, as seen in blushing, which is simply dilatation of the facial vessels.

Subsidiary Centres

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 are able to take the place of the great primary centre. 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 of the lower part of the spinal cord causes renewed vasomotor paralysis. In frogs this recovery takes place rapidly, the centres being less confined to the medulla than is the case in the more highly organized animals, but in the rabbit and dog it has been observed to occur more slowly.

Besides keeping up the normal tone, the arterial nervous mechanisms have the function of regulating the amount of blood supplied to various organs or parts at different times. Both vasomotor and dilator or inhibitory impulses are probably employed for this purpose.