The effect of this power of the nervous system is to give it a certain control over the circulation in particular parts.  Thus, though the force of the heart and the general average blood-pressure remain the same, the state of the circulation may be very different in different parts of the body.  The importance of this local control over the circulation is of the utmost significance.  Thus an organ at work needs to be more richly supplied with blood than when at rest.  For example, when the salivary glands need to secrete saliva, and the stomach to pour out gastric juice, the arteries that supply these organs are dilated, and so the parts are flushed with an extra supply of blood, and thus are aroused to greater activity.

Again, the ordinary supply of blood to a part may be lessened, so that the organ is reduced to a state of inactivity, as occurs in the case of the brain during sleep.  We have in the act of blushing a visible example of sudden enlargement of the smaller arteries of the face and neck, called forth by some mental emotion which acts on the vaso-motor center and diminishes its activity.  The reverse condition occurs in the act of turning pale.  Then the result of the mental emotion is to cause the vaso-motor nerves to exercise a more powerful control over the capillaries, thereby closing them, and thus shutting off the flow of blood.

Experiment 91.  Hold up the ear of a white rabbit against the light while the animal is kept quiet and not alarmed.  The red central artery can be seen coursing along the translucent organ, giving off branches which by subdivision become too small to be separately visible, and the whole ear has a pink color and is warm from the abundant blood flowing through it.  Attentive observation will show also that the caliber of the main artery is not constant; at somewhat irregular periods of a minute or more it dilates and contracts a little.

[Illustration:  Fig. 77.—­Some of the Principal Organs of the Chest and Abdomen. (Blood vessels on the left, muscles on the right.)]

In brief, all over the body, the nervous system, by its vaso-motor centers, is always supervising and regulating the distribution of blood in the body, sending now more and now less to this or that part.

[Illustration:  Fig. 78.—­Capillary Blood-Vessels in the Web of a Frog’s Foot, as seen with the Microscope.]

196.  The Pulse.  When the finger is placed on any part of the body where an artery is located near the surface, as, for example, on the radial artery near the wrist, there is felt an intermittent pressure, throbbing with every beat of the heart.  This movement, frequently visible to the eye, is the result of the alternate expansion of the artery by the wave of blood, and the recoil of the arterial walls by their elasticity.  In other words, it is the wave produced by throwing a mass of blood into the arteries already full.  The blood-wave strikes upon the elastic walls of the arteries, causing an increased distention, followed at once by contraction.  This regular dilatation and rigidity of the elastic artery answering to the beats of the heart, is known as the pulse.