5^0 THE SENSES AND SENSORY ORGANS. 



uuconic 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 

 (Xci'vcn- odcr 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 fibrillse 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 (Aiisschciclnug), 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 

 niL-rc sheathing organs, like the sarcolemma of muscle or the 

 sheath of a nerve.' 



Grenacher terms tin.- whole of his percipient cells a retinula, 



