5 8 - INVESTIGATION OF THE EYE. [sect. 230. 



are easily seen, the latter at the edge of a folded piece of the membrane, and 

 on carefully detached portions. A fresh eye is necessary for the examination 

 of the ciliary muscle, as its elements soon cease to be recognisable. The 

 muscles of the iris are to be studied in a blue eye, especially in that of 

 a child after the removal of the posterior pigment ; also in the eyes of white 

 rabbits, in which the sphincter pupilloe is readily seen without further trouble 

 on the addition of acetic acid. The same preparation may be employed for 

 the examination of the nerves of the iris, but a perfectly fresh eye and 

 diluted caustic soda are indispensable conditions in the investigation. Many 

 observations on the uvea are facilitated by blanching it with chlorine-water, 

 as is recommended by Wittich. The retina must be examined, when fresh, 

 from the surface, upon perpendicular sections, and upon the borders of 

 folds; it should be studied first with aqueous humour, and without the 

 employment of a covering-glass ; slight compression and teasing of it out 

 may also be practised in its subsequent examination. Chromic acid pre- 

 parations are here very important. Chromic acid, indeed, partially alters 

 the rods, but by no means invariably, and it preserves other parts so much 

 the better ; indeed, without this re-agent, Midler and myself could never 

 have arrived at the results above given, although Hannover erroneously 

 objects to its use on account of its action on the rods. The best way of using 

 chromic acid is to apply it immediately to a perfectly fresh retina, and to 

 follow all the stages of its action step by step. If a very diluted solution be 

 employed, the elements are very little altered, and can be readily isolated ; 

 if the solution is somewhat concentrated, sections can be made, more espe- 

 cially through the retina ; and without these, no complete view of the 

 .structure of this membrane can be obtained. I make them by spreading 

 out a piece of retina upon a slip of glass, with a solution of chromic acid, in 

 such a manner that it lies flat and cannot float. These sections are to 

 be made as fine as possible with a sharp convex scalpel or razor, and 

 they should be taken from the edge of a eut surface by pressure from 

 above ; this is easily done after a little practice. It is good, however, to 

 guide the cutting scalpel by means of a scalpel-handle brought under it 

 with the other hand, until it is immediately over the margin of the retina. 

 Such sections are more especially to be made in the region of the yellow 

 spot, and afterwards in other places, both in a longitudinal and transverse 

 direction ; when successful, they should present but a few layers of the 

 elements. The individual laminae are very sharply marked off from each 

 other, and they are first to be studied on these sections ; afterwards the 

 several layers may be carefully teased out or rendered more transparent with 

 caustic soda ; though this re-agent is not generally of much use, because it 

 causes the elements to become exceedingly pale. The hyaloid membrane is ' 

 always easily detached posteriorly from the retina along with the vitreous 

 body, and may be recognised in every eye on sections of the surface of the 

 vitreous body under the microscope ; in its folds it may be partially dis- 

 tinguished by the naked eye. The zonula Zinnii, on the other hand, is, in 

 fresh eyes, always covered by detached pigment and colourless epithelium 

 from the ciliary processes, and at its posterior end it is overlapped by the 

 retina, so that it cannot be well seen here ; but it is readily traced at its free 

 anterior part. Nevertheless, without other preparation than removing as 

 much as possible of the adhering structures with a brush, pretty distinct 





