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 
asheath. 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 always 
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, er), 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 which 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 
