ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 55 



The lens and the corneas form the anterior, dioptric part of the eye. 

 The posterior portion is a thick concave disc, completely inclosed in a 

 membranous " ommateal " sac. The anterior wall or septal membrane pro- 

 tects the ends of the retinal cells and forms an elastic cushion for the lens. 

 The still thicker inner wall forms the tough double sclerotica. The cells 

 within the sac form a closed vesicle with obliterated central cavity. The 

 originally simple wall consists of four layers, — (a) anteriorly, an outer 

 ganglionic layer, an inner ganglionic layer, the retinophorae, and the rods 

 containing the retinidia, and (&) posteriorly, an outer vitreous network, a 

 double argentea, and the red tapetum. (1) The retinophorse form the 

 largest and most important layer of the retina. Their outer ends, narrowed 

 to nerve-fibres, are attached to the periphery of the retina, whence they are 

 directed inwards towards the optic axis. Their structure and disposition 

 is described. A delicate structureless wall separates the retinophorse from 

 their rods. (2) The rods are columnar bodies of a faint yellowish-red 

 colour. They consist of a hyaline, refractive cap, surrounding a pyramidal 

 core, filled with a watery, non-refractive fluid. The course of the axial 

 nerve-fibre of the retinophor^, and the disposition of the radiating fibrill^ 

 which form the greater part of the rods, are then noted. (3, 4) Two 

 other groups of cells, the inner and the outer ganglionic layers, add to the 

 complication of the outer wall of the optic vesicle. The four strata are 

 modifications of a single layer. 



As to the inner wall, (1) the vitreous networle has hitherto been over- 

 looked. It is a very thin layer of hyaline substance, perforated by large 

 holes in which the inner ends of the rods fit. It is a cuticular secretion of 

 the outer layer of the argentea, and homologous with the cuticular rods 

 secreted by the retinophorse. (2, 3) The argentea is formed by the modifi- 

 cation of two cell-layers into refractive, laminated membranes, each composed 

 of minute square plates with bevelled edges. It is thicker in the centre 

 of the eye, gradually diminishing peripherally to a thin layer. " While 

 acting as a perfect reflector for incident rays passing through the lens, 

 it offers no great impediment to the entrance of light into the retina, after 

 passing through the colourless eye-stalk and red tapetum." (4) The 

 tapetum or " red pigment layer " consists of a single layer of cells, 

 decreasing in thickness peripherally, and ending with the argentea, where 

 the fibres from the axial branch of the optic nerve enter the retina. 



The optic vesicle, with the above eight layers, is contained in the omma- 

 teal sac. The anterior wall or septum of the latter forms a stout double 

 membrane on which the lens rests. The inner wall or sclerotica consists of 

 tough hyaline connective tissue, decreasing in thickness to the periphery 

 of the retina, where it becomes continuous with the septal membrane. It is 

 also a double layer. 



The optic nerve, which arises from the circumpallial, extends through 

 the centre of the stalk, and divides into two nearly equal branches. " The 

 basal or axial branch abuts against the sclerotica, and losing its sheath, 

 divides into many bimdles of free nerve-fibres, which, clinging closely to 

 the sclerotica, ascend radially towards the periphery of the retina, where 

 they penetrate, in quite distinct groups, the ommateal membrane, and 

 become continuous with the attenuated ends of the retinophorse, through 

 the centre of which they are extended as axial nerve-fibres. The lateral 

 or ganglionic branch ascends towards the shell side of the retina, over 

 which it is bent nearly at right angles, and is continued over the surface of 

 the septum, the thick outer layer of which it penetrates just below the 

 inner surface of the lens. Its fibres unite with the ganglionic layers, or 

 pass between their cells to the surface of the rods." 



