This mechanism is different in the case of glands and some other structures which, when active, require an abundant supply of blood. Such organs are provided with vaso-dilator in addition to vaso-constrictor nerves. The most conspicuous example of this is to be seen in the case of the submaxillary gland. The nerve to this gland runs for some distance as an isolated thread—the chorda tympani. Stimulation of the chorda tympani has the double effect of dilating the arteries of the gland and of causing it to secrete. But the administration of atropin prevents secretion. Vaso-dilation is then the only visible effect. Stimulation may increase sixfold the outflow of blood from the veins of the gland. It rushes through with such rapidity that it retains its bright arterial hue. The gland also receives a twig from the sympathetic cord in the neck, which, as already stated, controls the vessels of the face. By stimulating the one nerve or the other the physiologist can at will increase or diminish the amount of blood flowing through the submaxillary gland. Stimulating any sensory nerve causes in a reflex manner an increased outflow of constrictor impulses from the centre in the medulla oblongata to all parts of the body, with the exception of the part to which the sensory nerve appertains. Its own constituency receives an increased supply of blood. It is not difficult to appreciate the importance of this double action. A part is injured. The restrictions placed upon its supply of blood are suspended. Lest its increased consumption should lead to a general fall in pressure, all other parts have their supply curtailed. The effect is even more pronounced than this. The whole blood-pressure is raised above its ordinary level. The flow of blood to the injured part is therefore greater than it would be were relaxation of its arteries the only change.