THE EYE. 



417 



rounded fibrils, about twenty-four to each rod (Hensen). The 

 fibrils are sharply differentiated from each other and have a slightly 

 spiral course; when seen in transverse section these outer limbs 

 do not appear to he round 

 (Schultze), although 



others hold them to 

 be perfectly rounded 

 (Hoffmann and others), 

 and that the loss of 

 the cylindrical form is 

 due to the methods of 

 treatment. According 

 to Merkel the longitu- 

 dinal striation is caused 

 by a canalisation of the 

 outer limb, which ac- 

 cording to him encloses 

 the processes of the pig- 

 mented epithelial layer ; 

 he is also of opinion that 

 the spiral appearance is 

 an artificial product. In 

 the latter opinion he is 

 probably wrong, as per- 

 fectly fresh rods exa- 

 mined in aqueous humour 

 show the same spiral 

 appearance (Hoffmann) : 

 against the canalisation 

 view others observe that 

 the longitudinal stria- 

 tion is most distinct near 

 the inner limit of the 

 outer limb, and that it 

 is impossible to conceive 

 that the processes of 

 the pig-ment-cells should 

 terminate with such extremely regular ends (Hoffmann). 



In the central part of the inner end of the outer limb 

 -is seen a dark point when the structures are examined in trans- 

 verse section (Ritter, Manz, Schiess, Schultze, and others). The 



E e 



s]iiml 



Various i^ieparations from the eye of the frog : 

 chiefl3' from the retina. 



1. Rod from retina in aqueous humour, showing 



striation. 



2. Tlirêe rods and one cone after treatment with osmic acid. 



3. 4, 5. Rods examined in recent state. 



6, 7. Inner segments of two rods after treatment with osmic 

 acid. 



a Outer limb. 



6 Inner limb. 



(■ Lenticular body. 



d Nucleus of outer nuclear layer. 



e External limiting membrane. 



8. Nuclear body from inner nucle;ir layer. 



9. Twin-cone. 



10. Sustentacular fibre of retina. 



11. Surface view of pigmented epithelium of retina. 



12. 13. Isolated pigmented cells of retina. 



14. Four pigmented cells, rods and cones, external limiting 



membrane, and part of outer nuclear layer attached. 



15. Two pigmented cells ; each showing three attached rods. 



16. Muscle-fibre from the iris. 



Figs. I, 2, 3, 4, 5, 6, 7, 8, 9 are magnified 500 times. Fig. 10, 

 300 times. Fig. 16, 400 times. All are copied from Hoff- 

 mann's figiu'es. 



Figs. 12, 13, 14, 15 are copied from Morano's figures ; Hart- 

 nack, Oc. II, Obj. g. 



