[Illustration: Fig. 12--Picking up the retina in order to tear it away from the entrance of the optic nerve.] [Illustration: Fig. 13--The lighter area is the field of iridescence of the choroid.] 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 eye. (“Physiology of the Senses,” McKendrick & Snodgrass, page 101.) [Illustration: Fig. 14--Excavated posterior half of the sclerotic.] 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 NERVE [Illustration: Fig. 15--Enlarged to show the entrance of the optic nerve. (Page 48.)] The 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 blade, the latter being preferable, cut the nerve in two longitudinally. (Fig. 15.) The cutting must be done with one movement, otherwise the nerve will be hacked, and will not make a good specimen. This specimen will show the way the nerve fibers are arranged. A cross section should be cut from the optic nerve of another eye, and then the two sections should be compared. The cross section will show the sheath of the nerve a little better than will the longitudinal section. [Illustration: Fig. 16--Showing ciliary processes and crystalline lens.] 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 HALF