THE NERVE DISTRIBUTION IN THE EYE OF PECTEN IRRADIANS, 475 



case some of the colored pigment may adhere in places to the argentea, while along 

 the edge of some of the pigment-cells clings the refractive fringe of argentea. The 

 vitreous network is not reticular, but a homogeneous fluid that assumes a vacuolated 

 appearance only when treated with certain hardening fluids, especially such as contain 

 acetic acid. 



III. GENERAL DESCRIPTION OF THE RETINA. 



The retina has somewhat the form of a concavo-convex disk. It is enclosed by 

 a retinal sac (ommateal sac) and divided into three unequal layers of cells by two 

 membranes disposed parallel to the anterior wall of the sac. The three layers of 

 cells are the anterior, middle, and posterior. The four membranes of the retina are 

 designated as the anterior or septal wall of the retinal sac (Fig. 1, mb. sep.), the 

 anterior limiting membrane (mb. a.), the median limiting membrane (mb. m.}, and 

 the posterior membrane. Four sets of nerve elements represent the chief morpho- 

 logical constituents of the retina. The rods, forming the posterior layer of cells 

 (Fig. 3, bac.), the bipolar cells (nl. b'pol.) lying in the middle layer; the external gan- 

 glionic cells (gn. ex.} in the anterior layer, and connected with the side branch of the 

 optic nerve; and the marginal ganglionic cells (gn. marg.) connected with the basal 

 optic nerve and lying around the margin of the retina. Other elements of the retina 

 are the anterior (Fig. 2, sst. a.) and the middle supporting cells (sst. ra.). 



IV. STRUCTURAL ELEMENTS OF THE RETINA. 



A rod (Fig. 2, bac.) consists of a nerve-cell whose small anterior end projects 

 slightly beyond the median limiting membrane and whose much elongated posterior 

 portion is tubular and terminated bluntly. This portion is encased in a hyaline 

 sheath and its end is capped by a homogeneous cuticular substance, that in methylen- 

 blue preparations seems like the matrix that separates the rods. In specimens 

 otherwise prepared this matrix appears to be granular like the cytoplasm of the median 

 supporting cells, a continuation of which it then appears to be. A small nucleus sur- 

 rounded by refractive protoplasm lies in the anterior end of the rod, and from this 

 an axial fibre surrounded by a layer of granular striations extends to the posterior 

 end. The rod in fact is a sensory nerve-cell with its sheathed axon. 



The bipolar cells extend from the median limiting membrane outwards toward 

 the margin of the retina and form curves that decrease in extent from the middle 



