436 MANIPULATION. 



Eyes. — There are many objects of great interest that may 

 be obtained from the eyes of various animals, especially when 

 injected; amongst these may be enumerated the structure of 

 the crystalline lens, the pigment, the ciliary processes, the 

 retina, and the membrane of Jacob ; but as the greater num- 

 ber of specimens cannot be well preserved, so as to show their 

 characteristic peculiarities, the present list will only include 

 those which do not alter by any of the different modes of 

 mounting. The structure of the crystalline lens in fish is best 

 seen after the lens itself has been hardened either by drying, 

 by boiling, or by long maceration in spirit; after having 

 peeled off the outside, the more dense interior will be found 

 to split up into concentric laminae, and each lamina will also 

 be found to be composed of an aggregation of toothed fibres ; 

 these are best seen when mounted in fluid, but if dyed, they 

 wiU show very well in balsam. The pigment is easily 

 obtained by opening a fresh eye under water, it may then be 

 detached as a separate layer, and portions of it floated on 

 glasses to dry, after which they may be mounted in balsam. 

 The ciliary processes are best seen when injected : they should 

 be mounted in a convenient form of cell, with fluid, and 

 viewed as opaque objects, with a power from thirty to 

 forty diameters. The retina should be examined in a very 

 fresh eye between glasses, and a little serum or aqueous 

 humour added, to allow the parts to be well displayed ; but 

 water must be avoided, as the nervous matter wiU be found to 

 be considerably altered by it ; the membrane of Jacob will 

 also require the same precautions to be adopted, but the 

 vascular layer of the retina, when injected, may be well seen 

 after having been dried. The following list wiU give the 

 names of a few of the eyes from which the most striking speci- 

 mens may be obtained : — 



