RETINA. 1 09 



1. The uvea, or pigment-cells of the retina, sending fine processes into the next layer. This 

 layer is often detached, and was formerly regarded as belonging to the choroid. 



2. The layer of rods and cones (Jacob's membrane). The rods are easily detected in groups, 

 and seen to be more numerous than the cones, which are much shorter, and not so easily 

 seen. 



3. At the lower end of the rods and cones a well-defined line, indicating the position of 

 the external limiting membrane. 



4. The outer nuclear layer, deeply stained red or blue consisting of spherical nuclei 

 arranged in several rows. It is thicker than the inner nuclear layer. 



5. The external granular or internuclear layer, a narrow layer. 



6. The inner miclear layer, also deeply stained red or blue and consisting of three or 

 four rows of spherical nuclei, larger than those of the outer layer. 



7. The inner granular layer, much broader than the outer layer of the same name. 



8. The cellular or ganglionic layer, consisting of a single row of large, well-marked multipolar 

 nerve-cells, each with a well-defined nucleus. 



9. The layer of optic nerve-fibres, merely indicated as fine threads, as they have no white 

 substance of Schwann. 



10. Most internally is the internal limiting membrane. From this at regular intervals arise, 

 by a broad-winged base, the fibres of Miiller (connective tissue), which proceed outwards in the 

 retina between the nerve-cells. They may be traced as far as layer number 6. In the inner- 

 most part of the retina, near the cellular layer, sections of large blood-vessels are to be found. 

 Each of these parts ought to be examined separately. (Indicate the general arrangement in 

 PI. XXVI., Fig. i). 



(H). The pigment-cells or uvea. These cells are cut vertically, and are apt to be detached. 

 They send fine processes into the next layer. 



Eods and Cones. Observe the rods, more numerous than the short club-shaped cones. 

 Each consists of an outer and an inner segment. Notice that the inner segment perforates 

 the outer limiting membrane. Study each of the layers in succession. Notice especially the 

 nerve-cells their large size, their well-defined nucleus and nucleolus, the former with a distinct 

 envelope around it ; their processes one central, which has a connection with one of the nerve- 

 fibres of the optic nerve, and several processes which proceed outwards and branch in the 

 inner granular layer. Trace the outward course of the fibres of M tiller. 



The retina of the pig is selected because it shows so well the rods and cones ; but it is ad- 

 visable to study also a section of the retina of an ox, because in it the ganglionic cells are 

 specially large and easily investigated. 



Eetina of the Frog in Osmic Acid. EXAMINATION (H). Tease out a small piece in 

 glycerine. The rods are very large, and so are easily seen. Both rods and cones consist of 

 an outer and an inner segment or member. In the fresh state the outer member is transparent, 

 bright and glistening, but it is rapidly blackened by the osmic acid, and so is easily recog- 

 nised. It not unfrequently shows a tendency to cleave transversely. Each inner segment is 

 pale, and more or less granular. They may be seen perforating the external limiting mem- 

 brane, and containing one of the nuclei of the external nuclear layer in their course. (Indicate 

 the rods and these segments in PI. XXVI., Fig. 2.) 



The Eetina of a Codfish. EXAMINATION (H). This is specially selected, because it shows 

 the cones so admirably. Select a cone, and observe its large size ; notice its short tapering 

 outer segment and its large conical inner one, and notice how readily it shows the transverse 



