Twelve nerves arise from the brain on either side of the middle line, and pass out through openings in the skull. With the exception of the two first and the eleventh, they all arise from the gray matter in the medulla oblongata. They are—

(1)  Olfactory,            (5)  Trifacial,        (9) Glossopharyngeal,

(2)   Optic,                  (6)  Abducens,    (10) Pneumogastric,

(3)   Motor Oculi,       (7)  Facial,          (11) Spinal Accessory,

(4)   Trochlear,           (8)  Auditory,     (12) Hypoglossal.

Fig. 24.—Cranial Nerves of Left Side.

First, or olfactory, is the special nerve of sense of smell. Its fibres pass through the holes in the ethmoid bone to the mucous membrane of the nose (sensory).

Second, or optic, is the special nerve of sight. It enters the orbit through the optic foramen, and is distributed to the interior of the eyeball (sensory).

Third, or motor oculi, enters the orbit through the sphenoidal fissure, and supplies all the muscles of the eye, except the superior oblique and external rectus (motor).

Fourth, or trochlear, enters the orbit with the motor oculi, and supplies the superior oblique muscle only (motor).

Fifth, or trifacial, or trigeminal, is the largest cranial nerve It has three divisions : (1) The ophthalmic, which leaves th6 cranium through the sphenoidal fissure, and sends branches to the skin of the forehead, the lachrymal gland, the eye, and the nose (sensory). (2) The superior maxillary leaves the cranium by the foramen rotundum ; it supplies the skin of the temple, cheek, nose, upper lip, upper teeth, and the mucous lining of the mouth and pharynx (sensory). (3) The inferior maxillary leaves the cranium through the foramen ovale ; it gives branches to the temple and external ear, the teeth, and lower jaw, and supplies the tongue with a nerve of taste (motor and sensory).

Sixth, or abducens, enters the orbit by the sphenoidal fissure, and supplies the external rectal muscles of the eye only (motor). The muscles of the eye are therefore supplied by the third, fourth, and sixth nerves, which all pass out through the sphenoidal fissure. The second, and a branch from the fifth, supply the eye with special and ordinary sensations. The eye, therefore, has five nerves.

Seventh, or facial, leaves the cranium by the stylo-mastoid foramen ; it supplies all the muscles of expression of the face, and some muscles of the neck and ear (motor).

Eighth, or auditory, is the special nerve of hearing, and is distributed only to the internal ear (sensory).

Ninth, or glosso-pharyngeal, is a nerve of taste, and also supplies the muscles and mucous membrane of the pharynx (sensory and motor).

Tenth, or pneumogastric, is a very important nerve ; it is also called the vagus or wandering nerve, because of its wide distribution, having a longer course than any other cranial nerve. It passes out from the skull with the glosso-pharyngeal and spinal accessory nerves through the jugular foramen. It supplies branches to the pharynx, larynx, heart, lungs, oesophagus, liver, and stomach (motor and sensory).

Eleventh, or spinal accessory, nerve consists of two parts, a spinal and an accessory. The spinal ascends from the spinal cord through the foramen magnum. The accessory is

connected with the vagus ; it supplies the trapezius and sterno-mastoid muscles (motor).

Twelfth, or hypoglossal, nerve leaves the cranium through the anterior condyloid foramen, and supplies the muscles of the tongue (motor).