This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The optic nerve is the nerve of sight. Interference with it produces blindness. The oculomotor or third nerve supplies all the muscles of the orbit except the external rectus and superior oblique. If paralyzed, the eye cannot be moved upward, inward, or to any extent downward. There will be ptosis of the upper lid from paralysis of the levator palpebrae, and dilatation of the pupil and paralysis of the accommodation of the eye. If the sixth or abducens is paralyzed, the eye cannot be turned outward. If the fourth or pathetic is paralyzed, the superior oblique fails to act, and the double vision produced is worse when the patient looks down, because it is normally a depressor muscle. The lachrymal, frontal, and nasal branches of the fifth are nerves of sensation, hence, in supra-orbital neuralgia and that affecting the nasal branch, pain is felt in the orbit at the inner angle of the eye and down the side of the nose.
On the interior of the eye, the expansion of the optic nerve forms the retina. The retina is divided into two lateral halves, each supplied by a corresponding half of the optic nerve. When the nerve reaches the optic chiasm it splits into two parts, one (internal fibres) going to the opposite side of the brain, and the other (external fibres) to the ganglia on the same side of the brain. Posterior to the chiasm, the nerve fibres form the optic tracts. The optic tracts, after leaving the chiasm, wind around the crura cerebri to the external geniculate bodies, thence they pass to the thalami and anterior corpora quadrigemina, and are continued backward into the cuneus lobule of the occipital lobe of the brain.
It will thus be seen that a lesion affecting any portion of the optic pathway posterior to the chiasm will produce blindness of one-half of the retina of both eyes on the side of the injury; a right-sided lesion will produce blindness of the right half of both retinae, and a lesion on the left side, blindness of the left half of both eyes. This is called he7nianopia. It is right lateral hemianopia if the right half of the visual fields is affected, and left lateral if the left sides are affected. Affections of the optic nerve produce total blindness of that eye if the whole nerve is involved. If only a part is involved, then a unilateral hemianopia may ensue. A bitemporal hemianopia may be caused by a tumor involving the anterior or middle portion of the chiasm. A binasal hemianopia requires a symmetrical lesion on the outer side of both optic nerves or tracts. A brain tumor located in the cuneus lobule would cause a lateral hemianopia of the same side, right or left, of both visual fields, hence sometimes called homonymous.
The eyelids are composed of five layers, viz: (1) skin, (2) subcutaneous tissue, (3) orbicularis palpebrarum muscle, (4) tarsal cartilage with the contained Meibomian glands, (5) the conjunctiva. The juncture of the two lids at each end is called the inner and outer canthus.
The skin of the lids is thin and the subcutaneous tissue loose and devoid of fat. For these reasons blood finds its way readily into the lids and shows plainly beneath the skin, constituting the familiar "black eye." The skin lends itself readily to plastic operations, as it is easily raised and the gap left can be readily closed. The blood supply of the lids is abundant, so that the flaps are well nourished and sloughing is not apt to occur. The folds in the skin run parallel to the edge of the lids, therefore the incisions should be made as much as possible in the same direction. The orbicularis palpebrarum muscle passes circularly over the lids and lies on the tarsal cartilage toward the edge of the lids and on the orbitotarsal ligament above. The so-called tarsal cartilage or plate is composed of dense connective tissue and contains no cartilage cells. It is attached externally by the external (lateral) palpebral ligament and internally by the internal (medial) palpebral ligament or tendo-oculi. This latter passes in front of the lachrymal sac. The tarsal plate is continued to the rim of the orbit by the orbitotarsal ligament or septum orbitale. The expansion of the levator palpebral muscle ends in the upper edge of the tarsal cartilage and sends some fibres to the tissues immediately in front. The orbitotarsal ligament and tarsal cartilage prevent the fat of the orbit from protruding and also act as a barrier to the exit of pus.
The tarsal cartilage contains the Meibomian glands. These can be seen in life, by everting the lid, as yellow streaks passing backward from the edge of the lids.
Frequently these glands become obstructed and their mucus contents dilate the gland, forming a cyst known as chalazion. Suppuration may occur and pus instead of mucus is then contained within them. The wall of these cysts is formed by fibrous tissue containing some of the epithelial cells of the glands; therefore, if an uninflamed cyst is simply opened and its contents expressed, it will soon reform. To prevent this recurrence, the lining membrane is curetted in order to remove the mucus-forming cells. The cyst may point and be opened either on the side of the skin or conjunctiva, preferably the latter.
The openings of the Meibomian ducts are on the inner edge of the lids where the conjunctiva joins the skin. At the outer edge of the lids are the cilice or eyelashes and connected with them are sebaceous and sweat glands. Infection of these glands produces a small abscess called a stye. As they are on the outer edge of the lids they tend to discharge anteriorly and not toward the conjunctiva.
Fig. 97. - Vertical section of upper eyelid of child. X 15. (Piersol).
The conjunctiva covers the outer surface of the eye and the inner surface of the lids. The fold where it passes from one to the other is called the fornix. The tarsal or palpebral conjunctiva adheres closely to the tarsus and as it is transparent the Meibomian glands can readily be seen through it. The ocular or bulbar con-junctiva is loosely adherent to the sclerotic coat and through it the conjunctival vessels, which move with it, can be seen. The straight vessels going toward the cornea do not move when the conjunctiva is moved, because they lie deeper and are attached to the sclera.