bone, results in loss of
sight in the eye on the same side. In cellulitis of the orbit,
intra-orbital tumour, gumma and aneurysm in the region of the
cavernous sinus, also, the optic nerve may be implicated.

Lesions implicating the cortical centre for sight in the occipital
lobe give rise to hemianopia--that is, loss of sight in the lateral
halves of the fields of vision of both eyes--colour-blindness,
subjective sensations of light and colour, and other eye symptoms.

Double optic neuritis, followed by optic atrophy, is one of the most
constant effects of the growth of a tumour within the skull, and is
not uncommon in cases of cerebral abscess and meningitis. Pressure on
the optic chiasma, for example by a tumour of the pituitary body, is
associated with bilateral temporal hemianopsia.

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 comp