PROCURING OBJECTS. 101 



rated mechanically, or by maceration in dilute hydrochloric 

 acid. 



EYES. Many objects of interest may be obtained from the 

 eyes of various animals ; as the crystalline lens, the pigment, 

 the ciliary processes, the retina, and the membrane of Jacob. 

 The structure of the crystalline lens in fish is best seen after 

 the lens itself has been hardened by drying, boiling, or 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 will 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 parts of it floated on slides 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 cell 

 with fluid, and viewed as opaque objects. The retina should 

 be examined from a very fresh eye, between glasses, and a 

 little serum or aqueous humor added, to allow the parts to 

 be well displayed ; but water must be avoided, as the nervous 

 matter will be considerably altered by it; the membrane of 

 Jacob will also require the same precautions, but the vascular 

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

 having been dried. 



For the dissection of the eye, the plaster mould, described in 

 the chapter on Dissecting Objects, will be found useful. The 

 eye may be fixed to the plaster by bent pins while it is yet 

 fluid, as otherwise it would not remain firm. Sometimes the 

 eye is frozen, to facilitate dissection. 



MUSCULAR FIBRE. Muscles are of two kinds, voluntary 

 and involuntary ; from their functions. The voluntary muscles 

 of all the vertebrata, and the articulate animals (as insects, 



9* 



