3*4 



ELEMENTS OF HISTOLOGY. [Chap, xxxix. 





The cones are shorter than the rods, the pointed 

 end of the former not reaching 

 much farther than the junction 

 between the outer and inner 

 members of the rods. 



In the macula lutea and 

 fovea centralis of man and 

 most mammals there are pre- 

 sent cones only, and towards 

 the peripheral portion of the 

 retina they gradually de- 

 crease in numbers; in the 

 peripheral part there are only 

 rods. But in birds the cones 

 exceed the rods everywhere. 



In the bat and mole the 

 macula lutea possesses no 

 cones, and in the owl, rat, 

 mouse, guinea-pig, and rabbit, 

 they are few and small. 



445. The outer members 

 of the rods (only) contain in 

 the fresh and living state a 

 peculiar diffuse purplish colour 

 (Leydig, Boll, Kuhne) : this 

 is the visual purple or Rhod- 

 opsin of Kuhne. When 

 exposed to sunlight it 

 passes through red, orange, 

 and yellow, and finally 

 disappears altogether be- 

 comes bleached. There is 

 no visual purple in the rods 

 of Rhinolophus hipposideros, 

 fowl and pigeon ; in those 

 retinae in which the cones 

 contain coloured globules (see 



Tig. 160. Vertical Section 

 through the Frog's Retina. 



i, The pijrmented epithelium 

 of the retina or tapetum 

 nigrtun ; ft, the outer mem- 

 bers of the rods, those of 

 coues between them ; c, tin 1 

 inner members of the rods 

 and cones; rf, Hmitnns ex- 

 terna ; c, the outer nuclei ; /, 

 the outer molecular layer; 'g, 

 the inner nuclei; h, the inner 

 molecular layer ; i. the nuclei 

 of the ganglion cells; j, the 

 nerve-Bores ; the pyramidal 

 extremities of the radial 

 fibres arc well shown. (Atlas.) 



