272 



THE ESSENTIALS OF HISTOLOGY. 



elements, but merely an interlacement of ramified fibrils (fig. 301). 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. 



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

 consists of hexagonal epithelium-cells (fig. 303), 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, 

 many of which are in the form of minute crystals, lie in the inner 



FIG. 305. VERTICAL SECTION THROUGH THE MACULA LUTEA AND FOVEA CENTRALIS ; 

 DIAGRAMMATIC (after M. Sclmltze). 



1, nerve-layer; 2, ganglionic layer; 3, inner molecular; 4, inner nuclear ; and 5, outer 

 molecular layers ; 6, outer nuclear layer, the inner part with only cone-fibres forming 

 the so-called external fibrous layer ; 7, cones and rods. 



part of the cell, and after prolonged exposure to the light they are 

 found extending along the cell-processes between the rods (Kiihne), 

 their function being probably connected with the restoration of the 

 purple colouring matter which has been bleached by the light. This 

 extension of the pigment is accompanied by a shortening of the cones 

 (Engelmann). 



Fibres of MuUer.The fibres of Miiller (fig. 297, fig. 301, J, and 

 fig. 304) are long stiff fibres which pass through several of the retinal 

 layers. Commencing at the inner surface of the retina by expanded 

 bases which unite with one another to form the so-called internal 

 limiting membrane (fig. 297), the fibres pass through all the layers in 

 succession, until they reach the outer nuclear layer. Here they branch 

 and expand into a sort of reticular tissue which serves to support the 



