By practising on an enucleated eye, one may gain considerable ability in the use of the ophthalmoscope, and also learn to recognize the blood-vessels and other important parts of the retina. To do this, the eye to be examined must be very fresh, for only in this condition will the cornea and lens be sufficiently clear to permit rays of light to enter the inside of the eye. However, since the pupil is oblong in shape, and often only a narrow slit--but several millimetres in diameter--the field presented for observation is a rather limited one. To increase the pupillary aperture, take a pin, and force the point through the cornea about three or four millimetres from the corneo-scleral junction, and at right angles to the direction of the parallel edges of the pupil. After the pin has been pushed through until it has reached to within a short distance (one millimetre) of the edge of the iris, carefully pick up the iris by raising the pin into a position perpendicular to the cornea, and force the pin further down into the eye. The pupil will have been enlarged on one side. Do the same thing on the opposite side, and at each extremity of the pupil. (Fig. 55.) [Illustration: Fig. 55--Showing one pin before the iris has been picked up and pulled back, and three pins after the iris has been picked up and pulled back.] [Illustration: Fig. 56--Showing method of gathering up the extrinsic tissues in order to get rid of the pucker in the cornea.] The pupil will now have been made square, and so large that no difficulty will be experienced in reflecting either light into the eye, or in examining the inside of the eye. Care must be taken not to lacerate the anterior surface of the lens when the iris is drawn back by the pins. Putting the pins into the cornea, and using them as levers with the point of entrance in the cornea as a fulcrum, will pucker the cornea considerably, and a good clear fundus cannot be obtained. This is easily overcome. Simply gather up all the tissues surrounding the eye, force them backward, and hold them firmly with the fingers of the left hand. (Fig. 56.) The right hand is then free to handle the skiascope or ophthalmoscope, so that the interior of the eye may be thoroughly examined. Another way to prepare an eye for ophthalmoscopic examination is as follows: Go to a slaughter house and procure a beef eye from an animal that has been killed but a few minutes previously. Placing the eye immediately into an 8 per cent. solution of cocaine and leaving it there for about an hour will dilate the pupil to such an extent that work with the ophthalmoscope will be made very easy. This, as indicated, can be done only with an eye that is very fresh. [Illustration: Fig. 57--Showing window cut in sclerotic, choroid, and retina.]