, right ventricle;
O, right auricle;
P, trunk of pulmonary artery;
R, aorta; S, cardiac nerves;
T, recurrent laryngeal nerve;
U, superior laryngeal nerve;
V, submaxillary ganglion;
W, lingual branch of the 5th nerve;
X, ophthalmic ganglion;
Y, motor oculi externus.
]

Hence, all these actions of the internal organs just mentioned that are
necessary to the maintenance of the animal life, and of the harmony which
must exist between them, are controlled by the sympathetic system. But for
this control, the heart would stop beating during sleep, digestion would
cease, and breathing would be suspended. Gentle irritation of these
nerves, induced by contact of food in the stomach, causes that organ to
begin the churning motion needed for digestion. Various mental emotions
also have a reflex action upon the sympathetic system. Thus, terror
dilates the pupils, fear acts upon the nerves of the small blood-vessels
of the face to produce pallor, and the sight of an accident, or even the
emotions produced by hearing of one, may excite nausea and vomiting.

The control of the blood-vessels, as has been stated (sec. 195), is
one of the special functions of the sympathetic system. Through the nerves
distributed to the muscular coats of the arteries, the caliber of these
vessels can be varied, so that at one moment they permit a large quantity
of blood to pass, and at another will contract so as to diminish the
supply. This, too, is beyond the control of the will, and is brought about
by the vaso-motor nerves of the sympathetic system through a reflex
arrangement, the center for which is the medulla oblongata.

284. Need of Rest. The life of the body, as has been emphasized in
the preceding chapters, is subject to constant waste going on every
moment, from the first breath of infancy to the last hour of old age. We
should speedily exhaust our life from this continual loss, but for its
constant renewal with fresh material. This exhaustion of life is increased
by exertion, and the process of repair is vastly promoted by rest. Thus,
while exercise is a duty, rest is equally imperative.

The eye, when exactingly used in fine work, should have frequent intervals
of rest in a few moments of darkness by closing the lids. The brain, when
urged by strenuous study, should have occasional seasons of rest by a
dash of cold water upon the forehead, and a brief walk with slow and deep
inspirations of fresh air. The muscles, l