(9) Layer of rods.
(10) Pigment-epithelium.
Fig. 41.—Retina and Optic Nerve of Petromyzon. (After Müller and Langerhans.)
On the left side the Müllerian fibres and pigment-epithelium are represented alone. The retina is divided into an epithelial part, C (the layer of visual rod-cells), and a neurodermal or cerebral part which is formed of, A, the ganglion of the optic nerve and, B, the ganglion of the retina. 1, int. limiting membrane; 2, int. molecular layer with its two layers of cells; 3, layer of optic nerve fibres; 4, int. nuclear layer; 5, double row of tangential fulcrum cells; 6, layer of terminal retinal fibres; 7, ext. nuclear layer; 8, ext. limiting membrane; 9, layer of rods; 10, layer of pigment-epithelium. D, axial cell layer (Axenstrang) in optic nerve. The layer 6 is drawn rather too thick.
He points out especially the peculiarity of layer (2) (2, Fig. [41]), the inner molecular, in which two rows of nuclei are arranged with great regularity, the one row closely touching the membrana limitans interna, the other at the inner boundary of the middle third of the molecular layer. Of these two rows of nuclei, he describes the innermost as composed almost entirely of large nuclei belonging to ganglion cells, while the outermost is composed mainly of distinctly smaller nuclei, which in staining and appearance appear to belong not to nerve-cells but to the true reticular tissue of the molecular layer.
He also draws special attention to the remarkable layer (5) (5, Fig. [41]), which is not found in the retina of the higher vertebrates, the cells of which, in his opinion, are of the nature of ganglion-cells.
W. Müller, in 1874, gave a most careful description of the eye of Ammocœtes and Petromyzon, and traced the development of the retina; the subsequent paper of Kohl does not add anything new, and his drawings are manifestly diagrams, and do not represent the appearances so accurately as Müller's illustrations. In the accompanying figure (Fig. [41]) I reproduce on the right-hand side Müller's picture of the retina of Petromyzon, but have drawn it, as in Langerhans' picture, at the place of entry of the optic nerve.
From his comparison of this retina with a large number of other vertebrate retinas, he comes to the conclusion that the retina of all vertebrates is divisible into
A. An ectodermal (epithelial) part consisting of the layer of the visual cells, and