248 



HISTOLOGY 



and reaching across the eye-sac, so that it separates the blood sinus in 

 front from the vitreous fluid behind. Its most important layer is the 

 layer of visual cells, and these are found on its proximal surface pointing 

 backward. This makes the eye an inverted form like that of the spider 

 and that of the scorpion. As we shall see later, this peculiarity is also true 

 of the human eye. 



The visual cells form a thick layer and are rather peculiarly arranged. 

 Their distal ends give off the long, heavy visual rods, each of which con- 

 tains an axial filament. Hesse describes two filaments in an occasional 

 rod of Pecten tigrinus. These rods form a very even and thick layer in the 

 section. The line at which they all take their origin from the visual cells 

 is straight and even, and marked by a set of fine plates in the substance 

 between the cells. These plates are parts of a large, continuous mem- 

 brane, the limiting membrane, which has many openings for the rods to 

 pass through. Miss Hyde has described these rods as separate cells 

 beginning at and separated from the visual cells by the limiting mem- 

 brane, and each with a nucleus of its own. Hesse does not find this 

 nucleus in several other species of Pecten, and in Spondylus, and the 

 writers could not find it in Pecten tenuicostus. 



The visual cells come from the sides of the retina, and by well-grad- 

 uated curves turn in a proximal course until they end on the limiting 

 membrane, where the rods are given off. Their nuclei are large and oval, 



and lying among them are 

 a number of slender susten- 

 tacular cells with long, thin 

 nuclei. These supporting 

 elements give off fibers 

 which seem to pass out- 

 ward and reach the layer of 

 the cells above them. 



Lying still outside the 

 layer of visual cells is a 

 single layer of stout, heavy 

 cells with large nuclei. They 

 are called the outer gan- 

 glion cells. This layer shows 

 a slight differentiation in 

 ordinary preparations, and 

 under proper methylene- 

 blue treatment it is seen 

 that the row as seen in 

 section is composed of alternate nerve and supporting cells. The 

 methylene blue shows that neuro-fibrils enter this layer from the lateral 



. f w lim. m. 



FIG. 220. Two outer ganglion cells and three visual 

 cells from the retina of Pecten irradians to show their 

 connection through nerve fibrils, nv. fi. These fibrils 

 form a spiral coil around the nuclei of the outer gan- 

 glion cells, and pass into the main visual cells, lim.m., 

 limiting membrane. (Modified from Miss HYDE.) 



