ase 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.

The pulse may be easily found at the wrist, the temple, and the inner side
of the ankle. The throb of the two carotid arteries may be plainly felt by
pressing the thumb and finger