« PreviousContinue »
the name of the cornea (Fig. 75, 1). The corneal portion of the case of the eyeball is more convex than the sclerotic portion, so that the whole form of the ball is such as would be produced by cutting off a segment from the
FIG. 75. HORIZONTAL SECTION OF THE EYeball.
1, cornea; 1', conjunctiva; 2, sclerotic; 2', sheath of optic nerve; 3, choroid; 3", rods and cones of the retina; 4, ciliary muscle; 4', circular portion of ciliary muscle; 5, ciliary process; 6, posterior chamber between 7, the iris and the suspensory ligament; 7', anterior chamber; 8, artery of retina in the centre of the optic nerve; 8', centre of blind spot; 8", macula lutea; 9, ora serrata (this is of course not seen in a section such as this, but is introduced to show its position); to, space behind the suspensory ligament (canal of Petit); 12, crystalline lens ; 13, vitreous humour; 14 marks the position of the ciliary ligament; a, optic axis (in the actual eye of which this is an exact copy, the yellow spot happened, curiously enough, not to be in the optic axis); b, line of equator of the eyeball.
front of a spheroid of the diameter of the sclerotic, and replacing this by a segment cut from a smaller, and consequently more convex, spheroid.
15. The corneo-sclerotic case of the eye is kept in shape by what are termed the humours-watery or semi-fluid substances, one of which, the aqueous humour (Fig. 75, 7'), which is hardly more than water holding a few organic and saline substances in solution, distends the corneal chamber of the eye, while the other, the vitreous (Fig. 75, 13), which is rather a delicate jelly than a regular fluid, keeps the sclerotic chamber full.
The two humours are separated by the very beautiful, transparent, doubly-convex crystalline lens (Fig. 75, 12), denser, and capable of refracting light more strongly than either of the humours. The crystalline lens is composed of fibres having a somewhat complex arrangement, and is highly elastic. It is more convex behind than in front, and it is kept in place by a delicate, but at the same time strong and elastic, membranous frame or suspensory ligament, which extends from the edges of the lens to what are termed the ciliary processes of the choroid coat (Figs. 75, 5, and 76, c). In the ordinary condition of the eye this ligament is kept tense, i.e. is stretched pretty tight, and the front part of the lens is consequently flattened against it.
16. This choroid coat (Fig. 75, 3) is a highly vascular membrane, in close contact with the sclerotic externally, and lined, internally, by a layer of small polygonal bodies containing much pigmentary matter, called pigment cells (Fig. 74). These pigment cells are separated from the vitreous humour by the retina only. The rods and cones of the latter are in immediate contact with them. The choroid lines every part of the sclerotic, except just where the optic nerve enters it at a point below, and to the inner side of the centre of the back of the eye; but when it reaches the front part of the sclerotic, its inner surface becomes raised up into a number of longitudinal ridges, with intervening depressions, like the crimped frills of a lady's dress, terminating within and in front by rounded ends, but passing, externally, into the iris. These ridges, which when viewed from behind seem to radiate on all sides from the lens (Figs. 76, c, and 75, 5), are the abovementioned ciliary processes.
17. The iris itself (Figs. 75, 7, and 76, a, b) is, as has been already said, a curtain with a round hole in the
of the spot rests upon the entrance of the optic nerve, it is not perceived, and hence this region of the retina is called the blind spot.
6. The impression made by light upon the retina not only remains during the whole period of the direct action of the light, but has a certain duration of its own, however short the time during which the light itself lasts. A flash of lightning is, practically, instantaneous, but the
FIG. 74.-PIGMENT CELLS FROM THE CHOROID COAT.
A. Branched pigment cells from the deep layer.
B. Pigment epithelium. a, seen in face; b, seen in profile; c, pigment granules.
sensation of light produced by that flash endures for an appreciable period. It is found, in fact, that a luminous impression lasts for about one-eighth of a second; whence it follows, that if any two luminous impressions are separated by a less interval, they are not distinguished from one another.
For this reason a "Catherine-wheel," or a lighted stick turned round very rapidly by the hand, appears as a circle of fire; and the spokes of a coach wheel at speed are not separately visible, but only appear as a sort of opacity, or film, within the tire of the wheel.
7. The excitability of the retina is readily exhausted. Thus, looking at a bright light rapidly renders the part of the retina on which the light falls, insensible; and on looking from the bright light towards a moderately-lighted surface, a dark spot, arising from a temporary blindness
of the retina in this part, appears in the field of view. If the bright light be of one colour, the part of the retina on which it falls becomes insensible to rays of that colour, but not to the other rays of the spectrum. This is the explanation of the appearance of what are called complementary colours. For example, if a bright red wafer be stuck upon a sheet of white paper, and steadily looked at for some time with one eye, when the eye is turned aside to the white paper a greenish spot will appear, of about the size and shape of the wafer. The red image has, in fact, fatigued the part of the retina on which it fell for red light, but has left it sensitive to the remaining coloured rays of which white light is composed. But we know that if from the variously coloured rays which make up the spectrum of white light we take away all the red rays, the remaining rays together make up a sort of green. So that, when white light falls upon this part, the red rays in the white light having no effect, the result of the operation of the others is a greenish hue. If the wafer be green, the complementary image, as it is called, is red.
8. In some persons, the retina appears to be affected in one and the same way by rays of light of various colours, or even of all colours. Such colour-blind persons are unable to distinguish between the leaves of a cherry-tree and its fruit by the colour of the two, and see no difference between blue and yellow cloth.
This peculiarity is simply unfortunate for most people, but it may be dangerous if unknowingly possessed by railway guards or sailors. It probably arises either from a defect in the retina, which renders that organ unable to respond to different kinds of luminous vibrations, and consequently insensible to red rays or yellow rays, &c., as the case may be, or it may proceed from some unusual absorptive power of the humours of the eye which prevents particular rays from reaching the retina; or the fault may lie in the brain itself.
9. The sensation of light may be excited by other causes than the impact of the vibrations of the luminiferous ether upon the retina, Thus, an electric shock sent through the eye, gives rise to the appearance of a flash of light and pressure on any part of the retina produces a luminous image, which lasts as long as the
middle, provided with circular and radiating unstriped muscular fibres, and capable of having its central aperture enlarged or diminished by the action of these fibres, the contraction of which, unlike that of other unstriped muscular fibres, is extremely rapid. The edges of the iris are firmly connected with the capsule of the eye, at the junction of the cornea and sclerotic, by the connective tissue which enters into the composition of the so-called ciliary ligament. Unstriped muscular fibres, having the same attachment in front, spread backwards on to the outer surface of the choroid, constituting the ciliary muscle (Fig. 75, 4). If these fibres contract, it is obvious that they will pull the choroid forwards; and as the frame, or suspensory ligament of the lens, is connected with the
FIG. 76.-VIEW OF FRONT HALF OF THE EYEBALL SEEN FROM BEHIND. a, circular fibres; b, radiating fibres of the iris; c, ciliary processes; d, choroid. The crystalline lens has been removed.
ciliary processes (which simply form the anterior termination of the choroid), this pulling forward of the choroid comes to the same thing as a relaxation of the tension of that suspensory ligament, which, as I have just said, like the lens itself, is highly elastic.
The iris does not hang down perpendicularly into the space between the front face of the crystalline lens and the posterior surface of the cornea, which is filled by