Stripe, s. A lineary variation of colour; a shred of a different colour; a weal, or discoloration made by a lash or blow; a blow, a lash.
Structure, s. Act of building, form, make; edifice, building.
Under the term Structure of the Eye, Mr. White, describing its anatomy and physiology, divides the subject into two parts, that is, the eye itself and its appendages. Under the latter head, he comprehends the eyelids, the muscles which move it, the eyelashes, the lachrymal gland, the puncta lachrymalia, and lachrymal duct, the caruncle of the eye, the haw, and the membrane named conjunctiva, and the muscles by which the eye is moved.
He then proceeds:—Having described the appendages, as they are termed, I shall proceed to a description of the eye itself, the structure and economy of which is most curious and interesting. It is said to be composed of coats and humours, and this perhaps is the best manner of considering it. The first coat that appears is the cornea, or glass of the eye, which forms the anterior part, and is beautifully transparent. It is not of a circular form, as in man, but of an irregular oval, or rather oblong form, when examined out of the socket; but in its natural situation in the living horse, that part which projects beyond the eyelids is of a regular oval, or rather of an oblong form, and corresponding in some degree with the form of the pupil. It forms a larger portion of the globe of the eye than in man, and, by its convexity, causes the rays of light which pass through it to converge towards the pupil. This convexity may be too great or too little, and in either case render vision somewhat imperfect, and cause starting. The convexity of the cornea is preserved by the fluid which it incloses, named aqueous humour. On puncturing the cornea this fluid escapes, and then the cornea becomes flat and wrinkled. On removing the cornea the iris appears, which is a thin delicate brown or blackish muscle, with an oblong hole in the centre, named pupil. The iris is composed of two orders of fibres; the one circular, which, by contracting, diminishes, and even closes the aperture in the centre, named pupil; the other radiated, which by contracting, opens or enlarges the aperture or pupil. The second humour of the eye is situated immediately behind the pupil, and is named crystalline humour, or crystalline lens. On taking it out it appears to be a solid and beautifully transparent double convex lens, the posterior surface of which is more convex than the anterior one. It is found to become denser and denser from the circumference to the centre, and the slightest pressure so deranges it as to lessen or destroy its transparency.
The crystalline humour may be considered as composed of numerous concavo-convex lenses, admirably fitted to each other; those of the largest size having their circumference or edge opposed to and nicely joined to each other, thus forming altogether a double convex lens. The point of union between the two largest lenses is embraced by a band of muscular fibres, disposed in a circular direction, and named the ciliary ligament. These, by contracting, assisted probably by the muscles of the eye, increase the convexity of the crystalline lens, or rather of the series of concavo-convex lenses, of which the eye is composed, in a manner so just and equal, as to adapt it to the distance of the object at which the animal is looking, while, by the relaxation of these muscular fibres, the convexity of the lens is diminished by its own elasticity. These changes take place with inconceivable rapidity and accuracy. There are other muscular fibres proceeding from the band of circular fibres named ciliary processes, in a straight, or rather radiated, direction, towards the second coat of the eye, named tunica choroides; these are so arranged as to be drawn into folds, by which disposition they are enabled to perform their office more easily, which is that of drawing the lens towards the optic nerve, and thereby increasing the intensity of vision, whereby the animal is enabled to see small objects distinctly. The crystalline lens is inclosed in a transparent capsule, which is not in contact with it, there being about one drop, as it is computed, of a transparent liquid interposed, which, from the anatomist’s name who first observed it, has been called ‘liquor Morgagnii.’
I have seen a case where the convexity of the lens has been so increased, by an unusual degree of contraction of the circular fibres, named ciliary ligament, that it burst the capsule, and was forced out of its situation. I found it lying with its posterior convex surface on the inferior margin of the iris; about half of it appearing in the anterior chamber of the eye, as it is termed. The transparency of the lens was not affected, at least, it appeared so to me, and I rather think the accident had occurred, just at the time I observed it, from twitching the horse violently. In cataract, a disorder in which the crystalline lens becomes opaque, it always becomes globular in its form from an irritable state of the band of circular fibres; the same effect may be observed in the circular fibres of the iris, causing the pupil to be small, even in a moderate light, while the inflammation is going on, which occasions the cataract; but when the opacity is such as to exclude the light from the posterior part of the eye, which contains the third or vitreous humour, with the choroid coat, retina, and optic nerve, then the circular fibres relax, and the radiated fibres draw up the iris somewhat irregularly, which adhering to the capsule of the opaque lens, the pupil remains permanently open.
All that part of the eye which is posterior to the iris, is chiefly occupied by the third, or vitreous humour, and it is in this humour that the crystalline lens is imbedded. The vitreous humour is perfectly transparent, and consists of a fluid, inclosed in numerous small transparent cells, all of which appear to be inclosed in one delicate transparent membrane, named tunica arachnoidea. If the vitreous humour is cut, by snipping it with scissors, a fluid, like water, drops from it freely, so that it appears to be nothing more than water, probably holding a little salt in solution, which escapes when the transparent cells are thus cut open.
That part of the vitreous humour in which the lens is imbedded is different from the other parts, and of the consistence of jelly. Immediately behind the ciliary ligament, as it is termed, the arachnoid coat may be inflated with a small blowpipe, and made to resemble a circular canal; this has been called from the name of the person who first observed it, the circular canal of Petit. It is supposed to be connected with the radiated fibres of the iris. I have seen the whole of this humour in the eye of a sheep that had an hydatid in the right ventricle of the brain, of the consistence of jelly.
It is now time to speak of the optic nerve, and its appendage, the retina, with the third coat of the eye, named tunica choroides. The retina is a delicate transparent membrane, which embraces the vitreous humour, and is supposed to be an expansion or production of the optic nerve, serving to receive the impressions of objects, in order that they may be conveyed by the optic nerve to the organ of vision, named thalamus nervi optici, or speaking of both organs, thalami nervorum opticorum. After death, the retina becomes opaque, and of a light grey colour. It is so delicate a membrane, that there is some difficulty in preserving it for exhibition unless the eye is quite fresh. Under the retina lies the choroid coat, which is nothing more than a plexus of blood vessels, covered with a mucus substance of different colours. In the human eye it is black, which is the cause of the human pupil, or apple of the eye, appearing black; but in the horse it is variegated with mucus of a purple, a blue, a green, and a black colour, which, blending together, causes the pupil to appear of a dark blue colour. This mixture of colours in the bottom of the eye, or choroid coat, has been named tappetum lucidum.
The last and most considerable coat of the eye is the sclerotic coat, which is a very strong thick membrane, including all the other coats and humours, except the cornea. The sclerotic coat forms the greater portion of the globe of the eye, and is intimately united towards the anterior part with the cornea, which may be viewed as bearing the same relation to the sclerotic coat, as the glass of a watch does to the case. By maceration in water the cornea separates from the sclerotic coat completely.