520 THE SENSES AND SENSORY ORGANS. 
euconic eye in this, that the four cells secrete a fluid, which is 
held in a funnel-shaped cavity surrounded by pigment cells, and 
form no true cone. 
Grenacher concludes that the cone is a refractive medium, 
and has nothing whatever to do with the conversion of light 
into a nerve stimulus; but he adds: ‘It is the nervous rod 
(Nerven- oder Sehstab) which is the true percipient structure.’ 
Leydig, however, at one time at least, thought otherwise,and 
regarded the cone as the true percipient element, and lately 
Patten [239] has gone back to Leydig’s view, and has described 
certain imaginary nerve fibrilla in the cone, the existence of 
which Parker [250], using all Patten’s methods, was unsuccess- 
ful in demonstrating. 
Grenacher, in support of his contention that the great rods 
are percipient organs, says (p. 76) : 
‘When the visual rod (Sehstab) has been hitherto spoken of, 
it is the inner, sensitive, highly refractive, rod-like axial struc- 
ture which is usually intended ; this is surrounded by a sheath, 
which is usually intensely pigmented, in which nuclei have been 
discovered. But in general too little attention has been paid 
to it. 
‘The whole so-called visual rod consists of a number of 
long cells lying parallel to each other; to each of these cells 
belongs a transparent secretion (4 usscheidung), which we shall 
call rods from their analogy with those of the simple eyes, 
although this appellation is not always appropriate from the 
form of the structure. Sometimes, but not often, these rods 
are sunk in the anterior ends of the cells, and then the rods are 
slightly isolated from each other. More generally they form 
the inner edges of the cells, or spread more or less over them, 
and unite and form the axial rod, or so-called visual rod. 
The cells to which the segments of this visual rod belong form 
the sheath. The fibres of the optic nerve enter these cells, 
and therefore it is clear that they are agents of vision, and not 
mere sheathing organs, like the sarcolemma of muscle or the 
sheath of a nerve.’ 
Grenacher terms the whole of his percipient cells a retinula, 
