224 



THE ESSENTIALS OF HISTOLOGY 



being longitudinally striated. The inner ends of the rod- and cone- 

 fibres are believed to be connected with processes from the inner 

 granules, and through these with the nerve-cells and nerve-fibres. In 

 birds, reptiles, and amphibia, a small oil-globule, often brightly 

 coloured red, yellow, or green, is found in the inner segment of each 

 cone, and other variations of structure are met with in animals. The 

 cones are most numerous at the back of the retina ; they are fewer in 

 number, and the rods are proportionally more numerous, towards the 

 anterior part. 



ill 



Fl<;. 260. Pl(lMENTET) EPITHELIUM OF THE 



HUMAN RETINA. (Highly magnified.) 



a, cells seen from the outer surface with clear 

 lines of intercellular substance between ; 6, two 

 cells seen in profile with fine offsets extending 

 inwards ; c, a cell still in connection with the 

 outer ends of the rods. 



FlG. 261. A FIBRE OK MiJLLER FHOM THE 

 HUMAN RETINA, ISOLATED. L pa 



6, base of the fibre : n, its nucleus ; m.e.l. mem- 

 brana limitans extertia ; e.m.l. external mo- 

 lecular layer. 



The pigmentary layer is the most external part of the retina. It is 

 formed of hexagonal epithelium-cells (fig. 260), which are smooth exter- 

 nally where they rest against the choroid, but are prolonged internally 

 into fine filaments which extend between the rods. The pigment -granules, 



