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Testing Eye Movements. Abnormal eye movements, and analysis of them for medical students.
MOVEMENTS OF THE EYEBALL
The ocular muscles are supplied by three nerves the oculo-motorius, or 3d nerve; the trochlearis, or 4th nerve; and the abducens, or 6th nerve. Spasm and paralysis of each of these affect the muscles which they supply, and produce corresponding changes in the position of the eye ball. Their action on the pupil will be subsequently con sidered.
I. Paralysis of the Ocular Nerves.
(a) Oculo-motorius, or 3rd Nerve. According as the para lysis is complete or incomplete, the whole, or only one or more of the following muscles are affected: (a.) Levator Palpebrte Superioris. Paralysis causes droop ing of the upper eye-lid ptosis.
(b.) Superior Rectus. The eyeball turns downwards and slightly outwards when this nerve is paralyzed, and there is in consequence diplopia or double vision, the result of the visual axis of the two eyes not being di rected to the same object. As this divergence does not exist when the eyeballs are both turned down wards, the patient instinctively carries the head well thrown back.
(c). Internal Rectus. Paralysis here gives rise to diver gent strabismus (squint), with diplopia, the eyeball being rotated outwards on account of the unopposed action of the external rectus.
(d) Inferior Rectus. The affected eye is, in paralysis of this muscle, directed upwards and slightly outwards, And there is diplopia except when the object lies above the level of the eyes.
* Strictly speaking, these ought to be considered among the motor functions, but for convenience they are introduced here.
(e.) Inferior Oblique. In paralysis of this muscle the eye ball is turned slightly downwards and inwards, but this condition is rarely observed, as paralysis of the the inferior oblique as an isolated affection is exceed ingly uncommon.
(f.) Ciliary Muscle. The effect of paralysis of this muscle is that the patient is unable to change the convexity of the anterior surface of the lens, or, in other words, to focus his eye for near objects.
When the 3d nerve is paralyzed as a whole all these ac tions combine, and the result is that the lid droops, the eye ball is drawn downwards and outwards, and protrudes from its socket, the pupil (as we shall presently see) is di lated and immobile, and there is defective power of accom modation.
(2. ) Nervus Trochlearis, or 4th Nerve, supplies the superior oblique muscle, and when that is paralyzed there is diplopia, the field of vision being moved downwards and outwards.
(3. ) Nervus Abducens, or 6th Nerve, supplies the external rectus, paralysis of which causes convergent strabismus, with consequent diplopia, there being no power of rotating the eyeball outwards beyond the middle line. This condi tion frequently gives rise to giddiness, nausea, and vomit ing.
Paralysis of the ocular nerves may be either central or peripheral. In the latter case the paralysis is usually more complete. Ocu.lar paralysis is common in locomotor ataxia, syphilis, hemiplegia, etc., and after diphtheria.
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