s, that is, impressions from below
upwards to the brain resulting in sensation or feeling. In the anterior
part of the medulla, pass the nerves for motor transmission, that is,
nerve influences from above downwards that shall result in muscular
contractions in some part of the body.

The medulla is also the seat of a number of reflex centers connected with
the influence of the nervous system on the blood-vessels, the movements of
the heart, of respiration, and of swallowing, and on the secretion of
saliva. This spot has been called the "vital knot." In the medulla also
are centers for coughing, vomiting, swallowing, and the dilatation of the
pupil of the eye. It is also in part the deep origin of many of the
'important cranial nerves.

[Illustration: Fig. 116.--Illustrating the General Arrangement of the
Nervous System. (Posterior view.)]

271. The Cranial Nerves. The cranial or cerebral nerves
consist of twelve pairs of nerves which pass from the brain through
different openings in the base of the skull, and are distributed over the
head and face, also to some parts of the trunk and certain internal
organs. These nerves proceed in pairs from the corresponding parts of each
side of the brain, chiefly to the organs of smell, taste, hearing, and
sight.

The cranial nerves are of three kinds: sensory, motor, and both
combined, _viz_., mixed.

Distribution and Functions of the Cranial Nerves. The cranial nerves
are thus arranged in pairs:

The first pair are the olfactory nerves, which pass down through
the ethmoid bone into the nasal cavities, and are spread over the inner
surface of the nose. They are sensory, and are the special nerves of
smell.

The second pair are the optic nerves, which, under the name of
the _optic tracts_, run down to the base of the brain, from which an optic
nerve passes to each eyeball. These are sensory nerves, and are devoted to
sight.

The third, fourth, and sixth pairs proceed to the muscles of the
eyes and control their movements. These are motor nerves, the movers of
the eye.

Each of the fifth pair of nerves is in three branches, and proceeds
mainly to the face. They are called tri-facial, and are mixed nerves,
partly sensory and partly motor. The first branch is purely sensory, and
gives sensibility to the eyeball. The second gives sensibility to the
nose, gums, and cheeks. The third (mixed) gives the special sensation of
taste on the front part of the tongue, and ordinary se