upper eyelid induces a continuous gentle current
of tears over the surface, which carry away any
foreign particle that may have been deposited on
it. The fluid then passes through two small open-
ings (termed the puncta lachrymalia) into the
canals; whence its further course into the lower
porltion of the nose is sufficiently obvious. The
conjunctiva (or mucous coat), which cover the
front of the eyeball and lines the inner surface
of the lids, passes down, and lines the canals, sac,
and duct, and is thus seen to be continuous with
the nasal mucous membrane, of which it. may be
regarded aa an offshoot or digital prolongation.
see Mucous Membranes.
We shall conclude this sketch of the anatomy of the human eye with a brief notice of the nerves going to this organ and its appendages. Into each orbit there enters a nerve of special sense — viz. the optic nerve; a nerve of ordinary sensation — viz. the ophthalmic branch of the fifth nerve; and certain nerves of motion going to the muscular tissues and regulating the movements of the various parts — viz. the third, fourth, and sixth nerves. As the optic tracts from which the optic nerves originate are noticed in the article Nervous System, we shall merely trace these nerves from their chiasma, or commissure, forward. This commissure results from the junction of the optic tracts of the two sides, and it is especially remarkable for the fact that it presents a partial decussation of the nervous fibres, the central fibres of each tract passing into the nerve of the opposite side, and crossing the corresponding fibres of the other tract; while the outermost fibres, which are much fewer in number than the central ones, pass to the optic nerve of the same side. In front of the commissure the nerves enter the optic foramen at the apex of the orbit, receive a sheath or investment from the dura mater, acquire increased firmness, and finally terminate in the retina. The peculiar mode of termination the optic nerves in the cup-like expansion of the retina, the impairment or loss of vision which follows any morbid affection of them, and the constant relation in size which is observed in comparative anatomy between them and the organs of vision, afford sufficient evidence that they are the proper conductors of visual impressions to the sensorium.
The first or ophthalmic division of the fifth or trifacial nerve sends branches to the skin of the eyelids and to the conjunctiva. That it is the nerve of ordinary sensation of the eye is sufficiently obvious from the following facts: (1) That in disease of this nerve in the human subject, it is not uncommon to find the eyeball totally insensible to every kind of stimulus (particles of dust, pungent vapors, etc.); and (2) that if the nerve be divided in the cranium (in one of the lower animals), similar insensibility results.
The most important of the nerves of motion of the eye is the third nerve, or motor oculi. It supplies with motor power the elevator of the upper eyelid and all the muscles of the globe, except the superior oblique and the external rectus muscle, and, in addition to this, it sends filaments to the iris and other muscular fibres within the eye. The application of an irritant (in vivisection experiments) to its trunk induces convulsive contraction of the principal muscles of the ball and of the iris; while division of the trunk occasions an extertnal squint, with palsy of the upper eyelid and fixed dilatation of the pupil. The squint is caused by the action of the external straight and the superior oblique muscles, while the other muscles are paralyzed by the operation. The normal motor action of the nerve upon the iris in causing contraction of the pupil is excited through the optic nerve, and affords a good illustration of reflex action, the stimulus of light falling upon the retina, and. through it, exciting that portion of the brain from which the third nerve takes its origin. This nerve clearly exerts a double influence in relation to vision: (1) It mainly controls the movements of the eyeball and the upper eyelid; and (2) from its connection with the muscular structures in the interior it regulates the amount of light that can enter the pupil, and probably takes part, in the adjusting power of the eye to various distances.
The fourth nerve supplies the superior oblique muscle with motor power, while the sixth nerve similarly regulates the movements of the exter- nal straight muscle — the only two muscles in the orbit which are not supplied by the third pair. Although not entitled to be termed a nerve of the orbit, the facial nerve deserves mention as sending a motor branch to the orbicularis muscle, by which the eye is closed.
Physiology. Having described the anatom- ical structure of the eye. Ave are able to pro- ceed to the consideration of the uses of the various parts of this organ. Assuming a general knowledge of the ordinary laws of geometrical optics (see Light; Lens; etc.). we will trace the course of the rays of light, proceeding from any luminous body through the different media on which they impinge. If a luminous object — as, for example, a lighted candle — be placed at about the ordinary distance of distinct vision (about 10 inches) from the front of the eye, some rays fall on the sclerotic, and, being reflected, take no part in vis- ion. The more central ones fall upon the cornea, and of these some also are reflected, giving to the surface of the eye its beautiful glistening appear- ance; while others pass through it, are converged by it and enter the aqueous humor, which prob- ably exerts no perceptible effect on their direc- tion. Those which fall on and pass through the outer or circumferential part of the cornea are stopped by the iris, and are either reflected or ab- sorbed by it; while those which fall upon its more central part pass through the pupil and are con- cerned in vision. In consequence of its refractive power, the rays passing through a comparatively large surface of the cornea are converged so as to pass through the relatively small pupil and im- pinge upon the lens, which, by the convexity of its surface and by its greater density toward the centre, very much increases the convergence of the rays passing through it. They then traverse the vitreous humor, whose principal use appears to be to afford support to the expanded retina, and are brought to a focus upon that tunic, forming there an exact but inverted image of the object.
This inversion of the image may be easily ex- hibited in the eye of a white rabbit or other albino animal, after removing the muscles, etc, from the bavk part of the globe. The flame of a candle held begore the cornea may be seen inverted at the back of the eye, increasing in size as the candle is brought near, diminishing as it retires, and always moving in a direction opposite to that of the flame.
