It is in reference to the structure of the retina and vitreous body of the complex eyes that I have found myself unable to come to the same conclusions as Claus and Schewiakoff. Since the work of the latter goes much further into the detail of the subject than does Claus’s paper, it will be sufficient for me to compare my results simply with those of Schewiakoff.
The latter finds that the retina is composed of two kinds of cells, corresponding to the supporting and sensory cells referred to in the description of the nerve ring. These he figures (’89, Taf. II, Figs. 12 and 13) as alternating regularly. The two kinds of cells differ as follows:
(1) Shape. The supporting cells like those referred to before, are cone-shaped, having a proximal fibrous process that runs into the underlying stratum of nerve fibres, and on the surface of the retina a broad distal pigmented termination. The sensory cells are spindle-shaped, the proximal processes becoming continuous with fibres of the underlying nervous mass, while the distal process runs up to the surface of the retina (the part toward the lens) in between the ends of the supporting cell. The two kinds of cells are accordingly designated as pigment and visual.
(2) Position of nucleus. This comes in as a corollary of the shape. The nuclei of the visual cells lie in the enlarged central part of the spindle-shape, and, therefore, at a lower level than the nuclei of the alternating pigment cells.
(3) Processes in the vitreous body. The distal processes of the spindle-shaped visual cells are continued through the vitreous body to the cells of the lens as rod-like visual fibres which lie in canals in the (supposedly) homogeneous vitreous body. The pigment cells on the other hand have no fibres passing from them through the vitreous body, but in the latter are situated cone-shaped masses of pigment whose bases rest upon the broad ends of the pigment cells without, however, being a part of the cell.
(4) Pigment. The distal ends of the pigment cells in the retina are strongly pigmented, as the name implies. The processes of the visual cells, which alternate with these, are pigmented likewise, but the pigment is not so abundant and lies in the periphery of the cell body, leaving free a highly refracting central axis.
If the relation of these cells to each other has been made sufficiently clear, it will be understood that, in accordance with Schewiakoff’s scheme of the structure, sections that cut the retinal cells transversely give very different appearances at different levels. A section through the very tops of the retinal cells, that is, the last section of the retina before striking the vitreous body, would show large polygonal areas of heavy pigment (the ends of the pigment cells), in between which would lie the much smaller, less pigmented, highly refracting ends of the visual cells (’89, Taf. II, Fig. 19). A section lower down in the retina, that is, more toward the centre of the club, would strike the low-lying enlarged central portion of the visual cells with their contained nuclei, and the smaller, proximal ends of the pigment cells. It would, therefore, give the reverse appearance from the preceding section, namely, that of large unpigmented (or but slightly pigmented) areas (the swollen bodies and nuclei of the spindle-shaped cells), and in between them smaller pigmented areas, the ends of the proximally tapering pigment cells (’89, Taf. II, Fig. 20). A section on the other side of the one first described, that is, one of the first through the vitreous body, would show pigment areas of the same size as the large ends of the pigment cells (the cone-shaped streaks of pigment in the vitreous body which according to Schewiakoff are associated with the pigment cell), and in between them the cross-sections of the rod-like processes from the visual cells, lying in canals in the clear homogeneous ground-substance of the vitreous body (’89, Taf. II, Fig. 18).
Let me give a resumé of Schewiakoff’s conception of the structure of the retina.
a. There is an alternation of pigment and visual cells, the nuclei of the spindle-shaped visual cells lying at a lower level than those of the cone-shaped pigment cells.
b. From the visual cells extend rod-like processes into the vitreous body, lying in canals in the latter.