[3] “Anatomy and Physiology of the Eye,” Brown & Zoethout. THE CHOROID Select an eye that has had a long part of the optic nerve left on it and place it into a 5 per cent. solution of formaldehyde. Leave it in that solution for from two to three weeks. That period of time in the fluid will be sufficient to permit the choroid to become sufficiently toughened and hardened. Leaving it in the solution longer than that length of time will not injure the eye in any way. [Illustration: Fig. 22--Showing how to puncture the cornea. (Page 62.)] [Illustration: Fig. 23--Removing the cornea. (Page 63.)] [Illustration: Fig. 24--Showing method of inserting the scalpel to separate the choroid from the sclerotic.] After having removed the eye from the formaldehyde, wash it for a few moments in running water. This will remove the preserving and hardening fluid from the surface, and will save the hands a little from the effects of the fluid. Next remove all the muscles and fatty tissues from the sclerotic. After that has been done, puncture the cornea with the pointed jaw of the scissors about 2 mm. from the corneo-scleral junction. (Fig. 22.) Then proceed to cut the cornea away, being careful not to lacerate the choroid or the iris. (Fig. 23.) The escaping aqueous humor will flow over the eye and make it very slippery, and, therefore, difficult to hold. Dip the eye in water, wash it, and then take it out and thoroughly dry it with a cloth. This procedure is absolutely necessary, and, if omitted, will surely result in the dropping of the eye about the time the work on the specimen is nearly finished. Insert the scalpel between the peripheral edge of the exposed iris and what is left of the cornea. With the back edge of the scalpel, gently loosen the choroid from the inner side of the corneo-scleral junction to which part it is not securely attached. (Fig. 24.) This requires only ordinary care, and but little skill other than that necessary to always keep the scalpel close to the inner surface of the sclerotic. When the choroid-iris edge has been detached from the inner side of the corneo-scleral junction, the weight of the contents of the second tunic will cause it to sag and give opportunity to easily separate, with the back edge of the scalpel, the choroid from the sclerotic for about a distance of from 8 to 10 mm. It has been the method in the past to force water through a blowpipe between the sclerotic and the choroid, in order to separate the attachments. It has also been the method to work under water when wishing to expose or isolate either the choroid or the retina. It is unnecessary to do either of these two things.