270.  The Medulla Oblongata.  This is the thick upper part of the spinal cord, lying within the cavity of the skull.  It is immediately under the cerebellum, and forms the connecting link between the brain and the spinal cord.  It is about an inch and a quarter long, and from one-half to three-fourths of an inch wide at its upper part.  The medulla oblongata consists, like the spinal cord, of columns of white fibers and masses of gray matter, but differently arranged.  The gray matter is broken up into masses which serve as centers of origin for various nerves.  The functions of the medulla oblongata are closely connected with the vital processes.  It is a great nerve tract for transmitting sensory and motor impressions, and also the seat of a number of centers for reflex actions of the highest importance to life.  Through the posterior part of the medulla the sensory impressions pass, 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.