The ocular muscles are divided into extrinsic and entrinsic muscles. The latter are the dila tator and the sphincter pupillae. There are six extrinsic muscles, four recti and two obliques. The four recti muscles and the superior obliques have their origin around the margin of the optic foramen (Fig. 5), and diverge as they come forward to form the muscular funnel. The four recti muscles are attached by short tendons, to the sclera, 7 to 9 mm. from the cornea (Fig. 4).
The oblique muscles have a more compli cated course. The superior oblique runs along the upper inner wall of the orbit to send its tendon through the trochlea and thence back ward to its insertion in the upper half of the eyeball behind the equator. The inferior oblique arises near the lower inner margin of the orbit anteriorly and goes backward to its insertion behind the equator about in the horizontal meridian. (Fig. 4).
The muscles are innervated by three nerves. The oculo-motor nerve supplies the internal, superior and inferior recti, and the inferior oblique. Also the levator palpebrae superioris and the two interior muscles of the eye, (sphincter pupillae and ciliary muscle) are innervated by it. The external rectus has the abducens nerve; the superior oblique the troch lear. The nuclei for these various nerves lie upon the floor of the fourth ventricle.
By the action of the extrinsic muscles we use both eyes synchronously and have binoc ular vision. Disturbance of the muscular bal ance causes diplopia, because images are thrown on non-identical parts of each retina. Ortho phoria is normal balance of the eye muscles. Strabismus, squint, is the condition in which only one eye fixes an object at a time. alysis of the ocular muscles is opththalmoplegia. The eyelids are in origin folds of skin (Fig. 2), which have pushed their way over the bulb to protect it. The eyebrow, super cilium, limits the upper lid, while the lower lid passes without any sharp line of demarkation into the cheek. The palpebral fissure separates the two lids. Temporarily the lids join each other at a sharp angle — external canthus. The inner canthus is of horse-shoe shape, embracing a small fleshy growth — the caruncle. The sur face of the lids next to the globe is lined by a mucous membrane — palpebral conjunctiva. When the eyelids are shut the entire conjunc tiva forms a closed sac. Each lid is supported by a dense connective tissue plate — the tarsus. The upper lid is elevated by the levator .palpe brae superioris, which has its origin at the apex of the orbit, and is inserted into the tarsus. The orbicularis palpebrarum closes the lids. This is a flat, cutaneous muscle surrounding the palpebral fissure in the form of a circle. Drooping of the upper lid is called ptosis.
The lacrimal gland, organ secreting the tears, is situated in a depression in the upper, outer wall of the orbit, near the margin. Smaller
accessory glands are located along the inner margin of the upper tarsus. The tears con tain but a small amount of solids, principally sodium chloride (hence salty tears). Psychic weeping occurs only in man. After removal of the gland the eye is kept moist by the secre tion of the conjunctiva. Through the puncta the tears pass into the canaliculi and thence into the lacrimal sac. The latter is contracted below the nasal duct, emptying into the inferior meatus of the nose. Only when the secretion is much increased does any great quantity of tears discharge into the nose. Inflammation of the tear sac is called dacryocystitis. At the free margin of the lids there is a narrow strip — inter-marginal space — where the conjunctiva and the skin merge. The anterior margin is rounded and has swinging from it the cilia. The posterior margin is very sharp. Just in front of it is a row of orifices, mouths of the meibomian glands — sebaceous glands — em bedded in the tarsal plates. They secrete oil to prevent the tears from running onto the cheeks. The modified sweat glands of Moll empty into the follicles of the lashes.
Comparative man the eve attains its highest state of development. The farther we descend the scale of animal life the simpler the eye and the more restricted its functions. In the lower orders there are eyes which distinguish between light and darkness only. In the higher orders, as the vertebrate, the eyes perceive more or less perfect images.
In many unicellular organizations—animals and plants, bacteria and protozoa — sensitiveness to light is a property of the body as a whole. In the more highly organized, multicellular animals certain cells have the function of re sponding to light, all others having lost it. The simplest form of visual organ consists of an epithelial cell connected with a nerve fibre. The cell has the power of transforming light into another form of energy that can be con ducted along the nerve fibres to a central nerve organ. The eye of man is essentially reducible to an enormous number of sensory epithelial cells united to form the retina and optic nerve. These latter are the necessary parts of the eye, with the formation of which the development of the eye begins. All other parts of the eye develop later, and are designed for nutrition, protection or optical purposes. Thus the cornea and sclera protect the delicate retina, while the chorioid nourishes it. The cornea and lens refract the light and the iris, by means of its pupil, regulates the amount of light. Man and the higher animals are endowed with two eyes. Most insects and some crustaceans have two complete eyes with a number of single supernumeray ones. Some mollusks have as high as several hundred eyes which lie along their mantle.