THE VISUAL PYRAMIDS. 121 



one of the facets of the cornea, while its summit lies in 

 the middle dark zone. Each of these visual pyramids 

 consists of an axial structure, the visual rod, invested by 

 a sheath. The latter extends inwards from the margin 

 of each facet of the cornea, and contains pigment in 

 two regions of its length, the intermediate space being 

 devoid of pigment. As the position of the pigmented 

 regions in relation to the length of the pyramid is alwaj'S 

 the same, the pigmented regions necessarily take the form 

 of two consecutive zones when the pyramids are in their 

 natural position. 



The visual rod consists of two parts, an external 

 crystalline cone (fig. 28, B, cr), and an internal striated 

 spindle {sp). The crystalline cone consists of a trans- 

 parent glassy-looking substance, which may be made to 

 split up longitudinally into four segments. Its inner end 

 narrows into a filament which traverses the outer white 

 zone, and, in the middle dark zone, thickens into a four- 

 sided spindle-shaped transparent body, which appears 

 transversely striated. The inner end of this striated 

 spindle narrows again, and becomes continuous with 

 nerve fibres wliich proceed from the surface of the optic 

 bulb. 



The exact mode of connection of the nerve -fibres with 

 the visual rods is not certainly made out, but it is pro- 

 bable that there is direct continuity of substance, and that 

 each rod is really the termination of a nerve fibre. 



Eyes having essentially the same structure as that of 



