TECHNIC. 475 



drops to 30 c.c. of water), the whole process taking place in the light. 

 Golgi's method may also be used, but the gold method is more certain. 

 The sclera is treated in a similar manner. 



The pigmentation of the vascular layer interferes with examina- 

 tion, and albinotic animals should therefore be selected ; or the pigment 

 may be removed from the previously fixed eyeball with hydrogen peroxid 

 or nascent chlorin. The latter method is applied exactly as in cases where 

 the removal of osmic acid is desired. 



The adult lens is sectioned with difficulty, as it becomes very 

 hard in all fixing fluids. The anterior capsule of the lens may be removed 

 from previously fixed specimens and examined by itself. The lens-fibers 

 are demonstrated by maceration in y^ alcohol (twenty-four hours) or in 

 strong nitric acid. Before immersion the lens-capsule is opened by a 

 puncture. 



The retina can rarely be kept unwrinkled in eyes that have been 

 fixed whole. The eyeball should therefore be opened in the fixing fluid 

 and the latter permitted to act internally ; or the external tunics are 

 removed, thereby enabling the fixing fluid to act externally. 



Ranvier recommends subjecting the eyes of smaller animals 

 (mouse, triton) for a quarter or half hour to the action of osmic acid 

 fumes (see p. 24), after which the eyes are opened in yi alcohol with 

 the scissors. At the end of three or four hours the posterior half of the 

 eye is stained for some time in picrocarmin (p. 44), then carried over 

 into \ C J C osmic acid for twelve hours, washed with water, treated with 

 alcohol, and cut. 



In osmic acid preparations the rod-nuclei show dark transverse bands, 

 a condition due to the fact that the end-regions of the nuclei stain more 

 deeply. 



The retina is a good object for differential staining, as, for instance, 

 with hematoxylin-eosin, hematoxylin -orange G, etc. The latter combina- 

 tion is particularly successful in staining the rod- and cone-ellipsoids. 

 The examination of tangential sections should not be omitted. 



With the retina the best results are obtained by means of Golgi's 

 method. Attention must be called to the fact that the supporting struc- 

 tures of the retina are more easily impregnated than the nervous elements, 

 and that the latter can be demonstrated to any extent only in very young 

 eyes. 



Ramon y Cajal (94) recommends the following method, modi- 

 fied after Golgi : After the removal of the vitreous humor the posterior 

 half of the eyeball is placed for one or two days in a mixture containing 

 2,% potassium bichromate 20 c.c. and i% osmic acid 5 or 6 c.c. The 

 pieces are then dried with tissue paper and placed in a 0.75% silver 

 nitrate solution for an equal length of time. Without washing, the pieces 

 are immersed for from twenty-four to thirty -six hours in a mixture con- 

 taining 3% potassium bichromate 20 c.c., and i% osmic acid 2 or 3 c.c., 

 and then again carried over into a 0.75% silver nitrate solution for 

 twenty-four hours. In order to prevent precipitation it is advisable to 

 roll up the retina before treating, and to cover it with a thin layer of a 

 thin celloidin solution, which prevents it from again unrolling. 



The methylene-blue method (p. 184) will also bring out the 

 nervous elements of the retina, although the results are not quite so satis- 

 factory as those obtained by Golgi's method. 



