142 ELEMENTARY BIOLOGY. [CHAP. 



3 to 4 hrs. in picric acid; wash and transfer them 

 to alcohol of increasing strengths. 



Finally, stain with borax-carmine, imbed and cut 

 into longitudinal vertical sections. Mount in Canada 

 balsam. 



i. Examine with a low power, the whole eyeball will 

 be seen in section. 



a. The sclerotic; a cartilaginous outer capsule. 



b. The choroid; recognizable by the straggling 

 nature of its pigment cells, which form a sort 

 of loose network. 



c. The retina; seen to be composed of a number 

 of layers, differentiated under the action of the 

 staining reagent. Note the pigment layer ; red- 

 dish brown and regularly disposed, immediately 

 internal to the choroid, one-third the thickness 

 of the whole. 



ii. Examine your thinnest section under a high power. 

 Work from within outwards ; the following will be 

 seen, in order of enumeration. 



a. The internal limiting membrane, a thin structure- 

 less layer. 



b. The nerve-fibre layer ; thin and granular, with 

 delicate fibres. 



[a. and ft. are sometimes difficult to make out.] 



c. The nerve-cell layer; composed mainly of large 

 irregular cells with round nuclei. From some, 

 branches can be traced into the next layer. 



d. The inner molecular layer; this is thicker than 

 any of the preceding, and has a finely 



