III. Oculo-Motor Nerve.—One or more of the branches of this nerve may be compressed by extravasated blood, or be contused and lacerated in fractures implicating the region of the sphenoidal fissure. Fixed dilatation of one pupil may result from pressure by blood-clot, without other functional disturbance of the nerve. A tumour or an aneurysm growing in this region also may press upon the nerve. Sometimes both nerves are involved—for example, in fracture implicating both sides of the anterior fossa, and in tumours, particularly gumma, growing in the region of the floor of the third ventricle. In lesions of the cerebral hemispheres the third nerve is frequently paralysed. Its cortical centre lies in close proximity to the centre for the face ([Fig. 179]).
The most prominent symptoms of complete paralysis are ptosis or drooping of the upper eyelid, lateral strabismus, and slight downward rotation of the eye with diplopia. There are also dilatation of the pupil from paralysis of the circular fibres of the iris, and loss of accommodation and reaction to light from paralysis of the ciliary muscle.
Paralysis of the muscle supplied by the third nerve is frequently associated with paralysis of other ocular muscles. When all the muscles of the eye are paralysed, the condition is known as “opthalmoplegia externa”; it is usually due to syphilitic disease in the floor of the third ventricle.
IV. The Trochlear or Patheticus Nerve, which supplies the superior oblique muscle, may suffer in the same way as the oculo-motor nerve. When it is paralysed, there is defective movement of the eye downward and medially, and the patient may complain of diplopia when he looks downward.
V. Trigeminal Nerve.—The most important surgical affection of this nerve is “trigeminal neuralgia,” which has already been described (Volume I., p. 373). One or other of the divisions of the nerve may be torn in fractures of the base of the skull, and there results anæsthesia in the area supplied by it. In fractures crossing the apex of the petrous portion of the temporal bone, the great and small superficial petrosal nerves may be ruptured, and the soft palate and uvula are paralysed and there is difficulty in swallowing; there are also painful sensations in the ear. When the ophthalmic division is implicated, the conjunctiva is rendered insensitive, and conjunctivitis, which may be followed by ulceration of the cornea, results from exposure to dust and other foreign bodies, which, on account of the anæsthetic condition of the eye, are allowed to remain and cause irritation.
VI. Abducens Nerve.—This nerve, which supplies the lateral rectus muscle, has the longest course within the skull of any of the cranial nerves. In spite of this fact, it is comparatively seldom torn in basal fractures; but it is prone to be pressed upon by tumours, gummas, or aneurysms in the region of the base of the brain. When it is paralysed, medial strabismus results.
VII. Facial Nerve.—Paralysis of the facial muscles, more or less complete, is the most characteristic symptom of lesions of this nerve.
Paralysis of the Cerebral Type.—When the fibres of the nerve are implicated in any part of their course between the cortical centre and the nucleus in the lower part of the pons, the paralysis is of the upper neurone (cerebral) type. It affects the side of the face opposite to that of the lesion, and the defective movement is more marked in the lower than in the upper half of the face.
This form of facial paralysis may be due to the pressure of an intra-cranial tumour, abscess, or hæmorrhage, or to degenerative processes in the cerebral tissue, and as a rule other cranial nerves are also affected. Its recognition is chiefly of diagnostic and localising importance.
Paralysis of the Peripheral Type.—When the trunk of the nerve is implicated between the pontine nucleus and its peripheral distribution, the paralysis is of the lower neurone (peripheral) type, the muscles on the same side as the lesion being flaccid and atrophied.