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one inverted. One upright figure is reflected from the front of the cornea, which acts as a convex mirror. The second proceeds from the front of the crystalline lens, which has the same effect; while the inverted image proceeds from the posterior face of the lens, which, being convex backwards, is, of course, concave forwards, and acts as a concave mirror.
Suppose the eye to be steadily fixed on a distant object, and then adjusted to a near one in the same line of vision, the position of the eyeball remaining unchanged. Then the upright image reflected from the surface of the cornea, and the inverted image from the back of the lens, will remain unchanged, though it is demonstrable that their size or apparent position must change if cither the cornea, or the back of the lens, alter either their form or
Illustrates the change in the form of the lens when adjusted to distant, B to near objects.
their position. But the second upright image, that reflected by the front face of the lens, does change both its size and its position; it comes forward and grows smaller, proving that the front face of the lens has become more convex. The change of form of the lens is, in fact, that represented in Fig. 77.
These may be regarded as the facts of adjustment with which all explanations of that process must accord. They at once exclude the hypotheses (1) that adjustment is the result of the compression of the ball of the eye by its muscles, which would cause a change in the form of the cornea; (2) that adjustment results from a shifting of the lens bodily, for its hinder face does not move; (3) that it results from the pressure of the iris upon the front face
of the lens, for under these circumstances the hinder face of the lens would not remain stationary. This last hypothesis is further negatived by the fact that adjustment takes place equally well when the iris is absent.
One other explanation remains, which is, in all probability, the true one, though not altogether devoid of difficulties. The lens, which is very elastic, is kept habitually in a state of tension by the elasticity of its suspensory ligament, and consequently has a flatter form than it would take if left to itself. If the ciliary muscle contracts, it must, as has been seen, relax that ligament, and thereby diminish its elastic tension upon the lens. The lens, consequently, will become more convex, returning to its former shape when the ciliary muscle ceases to contract, and allows the choroid to return to its ordinary place.
If this be the true explanation of adjustment, the sense of effort we feel must arise from the contraction of the ciliary muscle.
22. Adjustment can take place only within a certain range, which admits of great individual variations. As a rule, no object which is brought within less than about ten inches of the eye can be seen distinctly without effort.
But many persons are born with the surface of the cornea more convex than usual, or with the refractive power of the eye increased in some other way; while, very generally, as age draws on, the cornea flattens. In the former case, objects at ordinary distances are seen indistinctly, because these images fall not on the retina, but in front of it; while, in the latter, the same indistinctness is the result of the rays of light striking upon the retina before they have been brought to a focus. The defect of the former, or short-sighted people, is amended by wearing concave glasses, which cause the rays to diverge; of the latter, or long-sighted people, by wearing convex glasses, which make the rays converge.
In the water camera the image brought to a focus on the screen at the back is inverted; the image of a tree for instance is seen with the roots upwards and the leaves and branches hanging downwards. The right of the image also corresponds with the left of the object and
vice versa. Exactly the same thing takes place in the eye with the image focussed on the retina. It too is inverted. (See Lesson X. § 11.)
23. The muscles which move the eyeball are altogether six in number-four straight muscles, or recti, and two oblique muscles, the obliqui (Fig. 78). The straight muscles are attached to the back of the orbit, round the edges of the hole through which the optic nerve passes, and run straight forward to their insertions into the scleroticone, the superior rectus, in the middle line above; one,
A, the muscles of the right eyeball viewed from above, and B of the left eyeball viewed from the outer side; S.R. the superior rectus; Inf. R. the inferior rectus; E.R., In. R. the external rectus; S. Ob. the superior oblique ; Inf.Ob. the inferior obliqu; Ch. the chiasma of the optic nerves (II.); III. the third nerve which supplies all the muscles except the superior oblique and the external rectus.
the inferior, opposite it below; and one half-way on each side, the external and internal recti. The eyeball is completely imbedded in fat behind and laterally; and these muscles turn it as on a cushion; the superior rectus inclining the axis of the eye upwards, the inferior downwards, the external outwards, the internal inwards.
The two oblique muscles are both attached on the outer side of the ball, and rather behind its centre; and they both pull in a direction from the point of attachment towards the inner side of the orbit-the lower, because
it arises here; the upper, because, though it arises along with the recti from the back of the orbit, yet, after passing forwards and becoming tendinous at the upper and inner corner of the orbit, it traverses a pulley-like loop of ligament, and then turns downwards and outwards to its insertion. The action of the oblique muscles is somewhat complicated, but their general tendency is to roll the eyeball on its axis, and pull it a little forward
24. The eyelids are folds of skin containing thin plates of cartilage, and fringed at their edges with hairs, the eyelashes, and with a series of small glands called Meibomian.
The front view of the rig! t eye dissected to show, Orb., the orbicular muscle of the eyelids; the pulley and insertion of the superior oblique, S.Ob., and the inferior oblique, Inf.Ob.; L.G. the lachrymal gland.
Circularly disposed fibres of striped muscle lie beneath the integuments of the eyelids, and constitute the orbicularis muscle which shuts them. The upper eyelid is raised by a special muscle, the levator of the upper lid, which arises at the back of the orbit and runs forwards to end in the lid.
The lower lid has no special depressor.
25. At the edge of the eyelids the integument becomes continuous with a delicate, vascular, and highly nervous mucous membrane, the conjunctiva, which lines the interior of the lids and the front of the eyeball, its epithelial layer being even continued over the cornea.
The numerous small ducts of a gland which is lodged in the orbit, on the outer side of the ball (Fig. 79, L.G.), the lachrymal gland, constantly pour its watery secretion into the interspace between the conjunctiva lining the upper eyelid and that covering the ball. On the inner side of the eye is a reddish fold, the caruncula lachrymalis, a sort of rudiment of that third eyelid which is to be found in many animals. Above and below, close to the caruncula, the edge of each eyelid presents a minute aperture (the punctum lachrymale), the opening of a
small canal. The canals from above and below converge and open into the lachrymal sac; the upper blind end of a duct (L.D., Fig. 80) which passes down from the orbit to the nose, opening below the inferior turbinal bone (Fig. 40, h). It is through this system of canals that the conjunctival mucous membrane is continuous with that of the nose; and it is by them that the secretion of the lachrymal canal is ordinarily carried away as fast as it forms.
But, under certain circumstances, as when the conjunctiva is irritated by pungent vapours, or when painful emotions arise in the mind, the secretion of the lachrymal gland exceeds the drainage power of the lachrymal duct, and the fluid, accumulating between the lids, at length overflows in the form of tears.