234 ELEMENTARY BIOLOGY. [CHAP. 



y. The intermediate mass; a plug of vacuolated con- 

 nective tissue, filling up the entire interspace be- 

 tween the optic bulb and the muscles. 



d. Examine your thinnest section with a high power. 

 Beginning at the exterior make out successively 



a. The corneal facet ; its flat outer and slightly ex- 

 cavated inner surface. 



ft. The crystalline cones. Cf. supra. Each is in 

 close apposition with the inner face of a corneal 

 facet. 



y. The striated spindles ; cf. supra. They are dis- 

 posed lineally with ft. 



8. The connective rod. Each is widest in front 

 where it joins the cone, but narrows posteriorly 

 where it is continuous with the striated spindle. 

 If fresh eyes be treated with osmic acid and then 

 teased out, each of these rods can be split up 

 into four fibres. 



e. The sheath of the above ; a nucleated investment 

 for each set of constituents. Pigment is de- 

 posited therein, being densest around the outer 

 ends of the cones and the greater portion of the 

 spindles, whence the appearance of the two black 

 zones seen under the low power. 



The layer of nerve fibres ; passing between y 

 and the optic bulb. Look for them in places 

 where the pigment may have fallen away. 



17. The optic bulb; mainly composed of delicate 

 fibres and small cells with large round nuclei, 

 partly scattered, partly aggregated into two oval 



