no 



however, on the ground of structure, the nucleus, and the 

 failure to prove a connection with nerve fibres, considers 

 them to be supporting cells. Until the difficulties of 

 following their fine fibrils are overcome, it will be better 

 to call them interstitial cells, which leaves the question 

 of function open. 



Argentea. — Underlying the retina is this layer (fig. 

 29, Arg., fig. 34), made up of refractive granules. This 

 is the layer which gives the eye the beautiful metallic 

 appearance, aided, of course, by the pigment layer — the 

 tapetum. Patten, by painting several white lines on the 

 base of the objective and focussing down on a large eye 

 removed from the mantle, was able to' see the image 

 formed in the eye. The different layers could be followed, 

 and the image was seen to be most distinct just before 

 reaching the tapetum and argentea, so that the lens and 

 argentea together act as a true optic lens and mirror, 

 and form an image just where the rods are placed to 

 receive it. 



The argentea (which does not generally stain in 

 sections), is made up of several laminae, and longitudinal 

 sections have the appearance of a series of layers of small 

 iridescent scales. If the argentea be looked at from the 

 surface (fig. 34), it is seen that the layers are formed of 

 numerous very small and almost square plates. These 

 are arranged regularly together in one plane, so that the 

 square face of each plate is in the plane of the argentea, 

 and the edges only are seen in longitudinal sections. The 

 argentea is thickest in the centre, where the laminae are 

 most numerous, and towards the sides it thins out. There 

 are no nuclei in this layer. 



Tapetum. — Below the argentea is this red pigment 

 layer (fig. 29, Taj).)- It is of considerable thickness in 

 the centre, but, like the argentea, decreases towards the 



