Fig. 135—Connection of spinal nerves with the cord. On the right is shown a nerve pathway from the skin to the muscle. A division of this pathway reaches the brain.
The Cranial Nerves.—From the under front surface of the brain, twelve pairs of nerves emerge and pass to the head, neck, and upper portions of the trunk. These, the cranial nerves, have names suggestive of their function or distribution and, in addition, are given numbers which indicate the order in which they leave the brain (Fig. 136). Unlike the spinal nerves, the cranial nerves present great variety among themselves, scarcely any two of them being alike in function or in their connection with different parts of the body. Several of them have to do with the special senses, and are for this reason very important. They[pg 297] connect the brain with the different parts of the head, neck, and trunk, as follows:
1. The first pair (olfactory nerves; nerves of smell; afferent) connect with the mucous membrane of the nostrils (Fig. 136).
2. The second pair (optic nerves; nerves of sight; afferent) connect with the retina of the eyes.
3. The third, fourth, and sixth pairs (motores oculi; control muscles of the eyes; efferent) connect with the internal and external muscles of the eyeballs (Fig. 136).
Fig. 136—Diagram suggesting the distribution and functions of the cranial nerves (Colton). See also Fig. 132.
4. The fifth pair (trigeminal nerves; nerves of feeling[pg 298] to the face, of taste to the front of the tongue, and of control of muscles of mastication; afferent and efferent) connect with the skin of the face, the mucous membrane of the mouth, the teeth, and the muscles of mastication.
5. The seventh pair (facial nerves; control muscles that give the facial expressions; efferent) connect with the muscles just beneath the skin of the face.
6. The eighth pair (auditory nerves; nerves of hearing; afferent) connect with the internal ear.