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The Cranial or Cerebral Nerves

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THE CRANIAL OR CEREBRAL NERVES.

There is a set of nerves, called the cranial or cerebral nerves, which pass from the brain through different openings in the skull, and are distributed over the head and face as well as to sonic parts of the trunk and certain of the internal organs. The nerves come off in pairs, one from corresponding parts of each side of the brain. Fig. 85, which represents the base of the brain, shows the places where these nerves issue from the brain substance. Refer ence should be made to that figure and to the description which accompanies it.

1. The first pair is the olfactory. They are bundles of very delicate nerve-filaments which conic off from the olfactory bulbs which lie under the front lobe of the cerebrum. They pass down into the nasal cavity through the ethmoid bone. They are nerves of the special sense of smell. (See Sect. XXII., SMELL.) 2. The second pair of nerves -is devoted to the sense of sight. They are the optic nerves, and pass forward into the cavity of the orbits to reach the eyeballs. (See Sect. XXII., SIGHT.) 3. The third pair are motor nerves, also pass ing into the orbits to supply the muscles of the eyeball. They are the nerves of motion for the muscles that turn the eye upwards, downwards, and inwards, and for the muscle that lifts the upper eyelid. They also supply the iris, that is, the circular curtain of the eye, by whose contractions the pupil of the eye is made larger or smaller. They leave unsupplied the muscle that turns the eye outwards, and another that turns it upwards and inwards. Thus, suppose this third nerve to be paralysed, the eye could not be turned upwards, or downwards, or in wards, but would be wholly under the influence of the two muscles already mentioned as inde pendent of the third nerve. The eye would thus, by these two muscles, be kept drawn downwards and outwards. The upper eyelid would also be paralysed, and would droop over the eyeball arid could not be lifted, while the pupil would be very large and immovable. It is obvious, therefore, that one eye would squint very much, and there would be double vision, two objects would be seen for one, owing to the want of united action between the two eyes.

4. The fourth pair of nerves is called the pathetic pair, because the nerves supply that one of the muscles, omitted by the third pair, whose contraction gives the upward turn to the eyes which we call pathetic.

5. Each one of this pair of nerves is in three divisions, and proceeds mainly to the face. It is therefore called trifacial. They are mixed nerves, partly sensory and partly motor. The first division, however, is purely sensory, and passes into the eyeball, on which it confers sensibility. The second division is also purely sensory, and gives sensation to the nose, and gums, and cheeks. The third division is partly

sensory and partly motor. Its sensory branches confer sensibility for taste on the front two thirds of the tongue, and ordinary sensation on the inner side of the cheeks and on the teeth, and also the scalp in front of the ear. Its motor branches supply the muscles of mastica tion (p. 113). Thus paralysis of the fifth pair of nerves would destroy the sensibility of the eyeball, and would, moreover, cause ulceration and inflammation to appear in it, would destroy to some extent the sense of smell and the sense of taste, would abolish sensation from the skin of the face, and would cause paralysis of the muscles of mastication, so that the movements of chewing would not be performed. If par alysis occurred to the nerve of one side only, that side of the face and tongue would be de prived of feeling, and the muscles of that side of the jaw being palsied the jaw would be wholly under the control of the opposite side, and would be pulled to the other side, so that the upper and lower teeth would not corre spond.

6. The sixth pair is called the abducent or abducting pair of nerves, because .they are motor nerves and supply the muscle that ab ducts or turns the eye outwards. This muscle was omitted by the third pair. Paralysis of the nerve leaves the eyeball under the influence of the opposite muscle, namely, that which turns it inward, so that the eye has an inward squint.

7. In each of the seventh pair of nerves there are two portions : the one called portio mollis (soft portion) is sensory, and is the auditory nerve—the nerve of hearing (see Sect. XXII., HEARING); the other, called portio dura (hard portion) or facial nerve, is motor, and supplies the muscles of expression. Thus paralysis of the auditory portion would produce deafness; paralysis of the facial would produce palsy of the muscles of the face. The result of facial palsy is that the affected side is smooth, un wrinkled, and motionless; the eyelids, being also palsied, are wide open and cannot be closed; and the face is wholly under the control of the muscles of the opposite side, which, having it all their on way, drag towards that side, so that the mouth is pulled away round. Refer to FACIAL PALSY, p. 177.

8. The eighth pair is very complex. The eighth nerve of each side consists of three trunks which arise from the medulla oblongata, and leave the brain by one opening.

The first trunk is called glossopharyngeal (glossa, the tongue, and phaumr, the throat) because it supplies the throat and back part of the tongue. It is partly sensory, conferring taste on the back part of the tongue, and partly motor.