For the study of the interior of the eye and its contents in situ either a fresh or a hardened eye will do; a hardened eye is preferable. In the dissection for isolating the hyaloid membrane, vitreous, lens, and other parts, the anterior and posterior halves of the evacuated eye may be separated entirely, and each half studied. However, the choroid and the retina will be more or less mutilated, and the vitreous and other parts will be removed. The absence of these parts will prevent one from receiving a definite idea of their anatomical relationships. Therefore, it is better to work with an entire and complete eye. Remove all the muscles and fatty tissues from the outside of the globe; then cut it in half through the equator, thus dividing it into an anterior and a posterior half. The cutting of the sclerotic, as well as the underlying tissues and the vitreous, should be done with the THE POSTERIOR HALFThe posterior half is taken first because it is the simplest and easiest of the two halves to dissect. In this half of the eye the retina may be readily seen through the vitreous; the choroid and its apparent iridescent colors through both vitreous and retina. (Fig. 10.) Sometimes the vitreous will adhere very closely to the retina. This occurs especially when the eye has been in formaldehyde for a long time. In such a case the removal of the vitreous without injuring the retina requires patience and care. The use of the scalpel and the scissors may become necessary. Another very good way to remove the vitreous is to take hold of the sclerotic, turn it so that the vitreous is downward, and then shake gently until the vitreous separates itself from the retina and, drops out. After the vitreous has been removed, notice its glassy appearance; hence its name—hyaloid body. Try to pull it apart with the fingers, and it will be noticed that it seems to be held together by more or less of a network of fibres. (Fig. 11.) Whichever method for removing the vitreous is followed, the retina will be left rather badly wrinkled and out of place. If the last-mentioned method, which is really the best of the Notice the thinness of the retina, and, also, that the seeming iridescence of the choroid shows through. The optic disc, which is the point of entrance of the optic nerve, and the optic cup are easily recognized, though neither will be seen as large as when viewed in the living eye with an ophthalmoscope. The blood vessels of the retina, as they ramify outward or forward, after their entrance through the optic nerve through which they pass, are also very plainly seen. A closer inspection will show, in the very centre of the “entrance” of the optic nerve, a whitish, pointed vessel, about 1 or 2 mm. long. That is the sloughed-off and atrophied end of the hyaloid artery, which, when the eye was in an embryonic state, ran forward from the central artery of the retina through the hyaloid canal to the posterior surface of the After the removal of the retina, the iridescence of the choroid (tapetum lucidum) (Fig. 13) may be examined with a hand lens, or, after its removal, a piece may be cut and placed under a microscope. This iridescence is, of course, not present in the human After the choroid is removed, which is accomplished in the same manner that the retina is removed, the inner side of the sclerotic is laid bare to view. The brownish color is mostly due to the presence of a small amount of pigment in the cells of one of the inner layers, it is also due, to a slight extent, to the staining influence of the perichoroidal fluid. (Fig. 14.) THE OPTIC NERVEThe excavated posterior half may be used now to show and to study the construction of the optic nerve. In cutting the optic nerve away from the sclerotic leave at least 5 mm. of the sclerotic attached. It will make handling easier. With the thumb and forefinger of the left hand hold the nerve in such a way on the table that it will be straightened out lengthwise, and then, using the scalpel or a safety-razor In cutting the longitudinal section, one is sometimes so fortunate as to cut through the central blood vessels of the retina. These vessels will show up then as a rather thin dark streak about 5 or 6 mm. long. THE ANTERIOR HALFThe anterior half will show the lens in situ, the ciliary processes, the posterior aspects of the iris and the lens, the corona ciliaris, the orbicularis ciliaris, and the ora serrata. (Fig. 16.) If the eye has been cut in two too far forward of its equator, the ora serrata will not be present. The ciliary processes and posterior aspect of the lens may be seen to better advantage when the anterior half of the vitreous is removed. This is done with the dull-pointed tweezers, by catching hold of the vitreous at any part of its free or cut margin, and stripping it off both the ciliary processes and the lens, using a prying, pulling movement to do so. (Fig. 17.) The two layers of the pigment cells, pars ciliaris retinae, which cover Now, remove the lens, using the point of the scalpel to cut through the suspensory ligament close to the lens. When this has been done there will be seen in the anterior chamber a thin, watery liquid—the aqueous humor. The corona ciliaris and orbicularis ciliaris may be better seen and studied if viewed through a hand lens. THE IRISTo see the iris, take hold of the cut edge of the choroid, and, gently pulling, separate it from its attachment to the corneo-scleral junction. The white ring on the anterior surface of this part of the second coat of the eye is the ciliary ring. With a scissors, cut around this ciliary ring at its outer edge. This specimen will show the anterior surface of the iris, and on the posterior side it will show the close relationship between the iris and the ciliary THE CORNEAAfter the anterior portion has had everything removed from it there will be left nothing but the first coat or tunic of the eye—the anterior portion of the sclerotic and the cornea. The way the cornea seems to fit into the sclerotic is not quite as one is led to believe when told that it fits into the sclerotic much the same way in which a watch crystal fits into a watch. By using the tweezers the cornea may be split. Nothing in the way of locating its layers can be recognized, however, unless a section is made for microscopic examination. The epithelial may be scraped off when the cornea is a trifle dry. This is the ocular epithelium reduced to a layer of flattened cells. THE CRYSTALLINE LENSIf the preceding dissections have been done, the crystalline lens will already have received some notice. To study the lens properly one should use an eye that has not been hardened and also an eye or the lens of an eye that has been in a 5 per cent. solution of formaldehyde for about two weeks. The lens in the unhardened eye will prove too friable to permit much handling. The dissection should be made, however, in order to give opportunity to notice the crystalline clearness of the lens substance, its great magnifying power, its attachments, its capsule, etc. For this purpose it is necessary to proceed only as in the dissection for the “hyaloid membrane, etc.” That is, use an eye that has been kept in a cool place for several days, and then open it, and remove hyaloid, vitreous, and lens intact, as in the first dissection taken up in this book. To examine the specimen in detail, turn it so the lens will be uppermost. (Fig. 18.) To remove the lens it is necessary to separate The tri-radiate lines on the posterior and the anterior surfaces of the lens will not be as clearly discernible as in the lens coming from the hardened eye. Close inspection and the use of a hand lens will help bring them out more clearly. Now, with the point of the scalpel try to separate the outer layers (cortex) from the It will be found that the lens after having been in the formaldehyde solution is no longer crystal like, but more or less translucent. When viewed from either the anterior side or the posterior side, the tri-radiate lines on each surface will be seen to begin at the poles of the lens and radiate outward toward the lens equator. Holding the lens up to a strong light will show that though the lines on either surface form angles of 120 degrees, the angles formed by the lines on one side with the lines on the other side are 60 degrees. On the anterior surface of the lens the vertical line extends upward from the pole; on the posterior surface downward from the pole. To study the laminated structure of the lens, it is best to boil the lens. The best way to do that is to drop the lens from either a hardened or unhardened eye into boiling water. Let it boil in the water for about two and a half to Insert the point of the scalpel carefully at one of the poles, and lift gently in the direction of one of the radiating lines. This will tend to raise one of the concentric layers, which can be easily peeled off. Repeat this in the direction of the other two radiating lines. Examining, with a hand lens, the exposed surfaces A lens that has been boiled and partly dissected A lens that has lost its transparency because of hardening in formaldehyde or boiling may be made clear and nearly transparent again in the following way: First: Place the lens in a 50 per cent. alcohol for several hours. Second: Remove the lens, and let it drain on a piece of blotting-paper; then place it in a 75 per cent. alcohol. Third: Remove the lens, as before, then place it in an 85 per cent. alcohol. The lens may be left in this alcohol from ten to twelve hours, after which length of time it should be removed and drained. Fourth: Place the lens in an absolute alcohol, and leave it there for ten or twelve hours. Several hours longer will not injure the lens, nor interfere with the success of the work. Fifth: Remove the lens from the absolute alcohol. Place it upon a piece of blotting-paper, moving it to another place on the blotting-paper whenever the paper around the lens seems to have taken Now drop the lens into xylol. Benzine will answer, though it will not produce quite so clear a lens as the xylol does. At the end of 24 or 36 hours the softer cortex will show quite clear, while the harder nucleus will be still cloudy. At the end of a week the whole lens, if it is a small one—pig, calf, sheep—will have become quite clear and transparent; if from a beef eye it will take longer. It sometimes takes nearly two weeks. In the case of a boiled lens Cedar oil may also be used for the purpose of clarifying or “clearing” the lens. Harden in the usual way, run through the alcohols, and then place in cedar oil. The oil, however, will stain the lens a yellowish brown, and the lens will not be as transparent and clear as when xylol is used. A. Lens hardened in formaldehyde. The longer a lens is left in either of these two clarifying fluids the harder and smaller it will become. At the end of a month or six |