AMBLYOPIA AND AMAUROSIS.

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When the optic nerve filaments become anasthetic, sight is impaired in the exact ratio of the loss of sensibility; hence we speak of the condition of "amblyopia," when the sight is partly destroyed by this condition, and of “amaurosis” when the sight is entirely destroyed.

We may consider a loss of sensibility of the optic nerve filaments as a symptom of the gravest import, since it indicates either some disease of the brain or some advanced changes of the nerve itself. The brain conditions which are most liable to produce this condition are as follows: neuroretinitis, which may follow cerebral hemorrhage, cerebral softening, Bright's disease, lead poisoning, and syphilis; the various forms of ataxia; cerebral tumors; chronic effusion into the ventricles; and hysterical cerebral disorders.

The local conditions which may result in optic anæsthesia include inflammation of the retina and the adjoining structures; hæmorrhage into the retina; retinal tumors; the compression of glaucoma; pressure of tumors, in the orbit or cranium, upon the optic tracts; thickening of the meninges in the vicinity of the optic chiasm; and traumatism.

Atrophy and sclerosis of the corpora geniculata may result in amaurosis'; lesions of the cerebellum may be accompanied by symptoms referable to the optic apparatus (probably on account of the pressure created upon adjoining regions of the encephalon); and an increase of intra-cranial pressure, from any cause, may produce retinal changes.

THE THIRD OR "MOTOR OCULI" NERVE.

This nerve has its apparent origin from the inner border of the crus cerebri. The deep origin of the nerve has been discussed in preceding pages, which treat of the crus cerebri ; and also in the introductory pages of this section, to which the reader is referred.

The course of this nerve, after it escapes from the brain, is of importance, from the relations which it has with impor

I See page 372 of this volume.

See page 235 of this volume.

tant structures, and from the physiological phenomena produced by it. It pierces the dura mater opposite to the an

FIG. 92.-Distribution of the motor oculi communis. (Hirschfeld.)

1, trunk of the motor oculi communis; 2, superior branch; 3, filaments which this branch sends to the superior rectus and the levator palpebri superioris; 4, branch to the internal rectus; 5, branch to the inferior rectus; 6, branch to the inferior oblique muscle; 7, branch to the lenticular ganglion; 8, motor oculi externus; 9, filaments of the motor oculi externus anastomosing with the sympathetic; 10, ciliary nerves.

terior clinoid process, in order

to reach the outer wall of the cavernous sinus, where it lies in close relation with the fourth cranial nerve, and the ophthalmic branch of the fifth cranial nerve, being above them both, and also with the cavernous sinus, which lies internal to it. It is in this region that the nerve is joined by filaments from the cavernous plexus of the sympathetic system.

The nerve now passes from the cavity of the cranium by means of the sphenoidal fissure, having, however, divided into two branches, before its

escape, called the superior and inferior.

In the sphenoidal fissure, these two branches are placed

between the two heads of the external rectus muscle of the eyeball, and from this point they pass onward to their respective distributions, viz., the superior branch to the levator palpebræ and the superior rectus muscles, and the inferior branch to the inferior oblique, the inferior rectus, and the internal rectus muscles, and, by a small filament, furnishing the motor root to the ciliary or lenticular ganglion of the orbit.

The third cranial nerve thus supplies all of the muscles of the eye but two, viz., the superior oblique and the external rectus muscles, which derive their motor power, respectively, from the fourth and the sixth nerves. It also supplies filaments to the ophthalmic ganglion (which is also called the ciliary, and the lenticular ganglion), which filaments

THE MOTOR OCULI NERVE.

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are subsequently distributed to the ciliary muscle and the iris.

It is now claimed that the fibers of the third nerve, which

pass to the aqueduct of Sylvius, decussate; and it is to this

FIG. 93. Ciliary muscle; magnified 10 diameters. (Sappey.)

1, 1, crystalline lens; 2, hyaloid membrane; 3, zone of Zinn; 4, iris; 5, 5, one of the ciliary processes; 6, 6, radiating fibers of the ciliary muscle; 7, section of the circular portion of the ciliary muscle; 8, venous plexus of the ciliary process; 9, 10, sclerotic coat; 11, 12, cornea; 13, epithelial layer of the cornea; 14, membrane of Descemet; 15, ligamentum iridis pectinatum; 16, epithelium of the membrane of Descemet; 17, union of the sclerotic coat with the cornea; 18, section of the canal of Schlemm.

anatomical arrangement of its fibers of origin that the effect of the pupil of one eye upon the condition of the pupil of the opposite eye is occasionally observed in disease, and that the muscles of the two eyes, as well as the iris, are thus enabled to work in perfect harmony with each other. As an example of this, it is occasionally observed that, when amaurosis affects one eye, the pupil of the diseased organ will not respond to the effect of light upon the retina of that side, but, when the

light creates a movement of the iris of the unimpaired eye, the pupil of the opposite side also responds, thus showing that reflex action is possible between the two eyes.

MECHANISM OF THE CONTRACTION OF THE PUPIL.

The mechanism of the reflex act, by which the third nerve is enabled to so affect the contraction of the pupil as to have its varying size correspond exactly to the requirements of the retina, as regards the amount of light

FIG. 94.-Choroid coat of the eye and the ciliary nerves. (Sappey.)

1, optic nerve; 2, 2, 2, 2, 3, 3, 3, 4, sclerotic coat, divided and turned back to show the choroid; 5, 5, 5, 5, the cornea, divided into four portions and turned back; 6, 6, canal of Schlemm; 7, external surface of the choroid, traversed by the ciliary nerves and one of the long ciliary arteries; 8, central vessel into which open the vasa vorticosa; 9, 9, 10, 10, choroid zone; 11, 11, ciliary nerves; 12, long ciliary artery; 13, 13, 13, 13, anterior ciliary arteries; 14, iris; 15, 15, vascular circle of the iris; 16, pupil. necessary for perfect vision at all times and under all circumstances, is a subject of interest to those who study anatomy from the standpoint of its physiological bearings. The optic nerve, when a person comes from darkness into the light, receives, on account of the dilated condition of the pupil, an excess of light which at once compels the eye to momentarily close' until the pupil shall become contracted. The

A reflex act produced through the optic nerve upon the orbicularis palpebrarum muscle.

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sensation of over-stimulation created in the optic nerve by the glare of light entering the dilated pupil is carried backward to the brain, and, probably in the region of the aqueduct of Sylvius, creates a reflex act which sends motor impulses along the fibers of the third nerve to the iris, by means of the branch to the ciliary ganglion. Thus it happens that, when the eye is again opened, the sensation of distress in the optic nerve is no longer present, and the pupil is found to be contracted in a direct proportion to the amount of light which at the time exists.

REASONS FOR THE PECULIAR DISTRIBUTION OF THE THIRD NERVE.

The distribution of the third cranial nerve may suggest to the inquiring mind the following questions: "Why does

FIG. 95.-Plexus of ciliary nerves.-Nerves of the iris. (After Sappey.) A, choroid; B, iris; 1, 1, 1, 1, ciliary nerves dividing at their terminal extremity into two or more branches, which anastomose to form a circular plexus surrounding the greater circumference of the iris; 2, 2, plexus formed by this anastomosis; 3, 3, nerves of the iris originating from this plexus.

Nature use three nerves to control the movements of the six ocular muscles, when she could have used one nerve to accomplish the effect? Why are the internal rectus, the inferior oblique, and the inferior rectus supplied from one nerve source, to the exclusion of the external rectus, and the supe