THE RETINA, OR TUNICA INTERN A 



1103 



an eminence, the optic papilla (colliculus nervi optici) ; the central portion is 

 depressed and is called the optic cup (excavatio papillae nervi optici). The arteria 

 centralis retinae pierces its centre. This is the only part of the surface of the 

 retina from which the power of vision is absent, and is termed the blind spot. 



OPTIC 



LAMINA CRIB ROSA PAPILLA 

 OF SC 



RETINA 

 CHOROID 



POSTERIOR 



SHORT CILIARY 



ARTERY AND 



VEIN 



PIAL 

 SHEATH 



DOI 



SHEJ 

 ARACH 



INTERVAGIN 



BUNDLES OF 

 OPTIC NERVE 



CENTRAL ARTERY AND 

 VEIN OF RETINA 



FIG. 811. The terminal portion of the optic nerve and its entrance into the eyeball, in horizontal section. 



(Toldt.) 



Structure. The retina is an exceedingly complex structure, and, when examined micro- 

 scopically by means of sections made perpendicularly to its surface, is found to consist of many 

 layers of nerve elements bound together and supported by the sustentacular fibres. The layers 

 of the retina are: 



1. Membrana limitans inter na. 



2. Layer of nerve fibres (stratum opticum). 



3. Ganglionic layer, consisting of nerve cells. 



4. Inner plexiform layer. 



5. Inner nuclear layer, or layer of inner granules. 



6. Outer plexiform layer. 



7. Outer nuclear layer, or layer of outer granules. 



8. Membrana limitans externa. 



9. Jacob's membrane (layer of rods and cones). 

 10. Pigmentary layer (tapetum nigrum). 



1 . The membr ana limitans interna is the most internal layer of the retina, and is in contact 

 with the hyaloid membrane of the vitreous humor. It is derived from the supporting frame- 

 work of the retina, with which tissue it will be described. 



2. The layer of nerve fibres is formed by the expansion of the optic nerve. This nerve passes 

 all the other laers of the retina, except the membrana limitans interna, to reach its 



destination. As the nerve passes through the lamina cribrosa of the scleral coat, the fibres of 

 which it is composed lose their myelin sheaths and are continued onward, through the choroid 

 and retina, as simple axones. When these amyelinic axones reach the internal surface of the 

 retina, they radiate from their point of entrance over the surface of the retina, become grouped 

 in bundles, and in many places arranged in plexuses. Most of the axones in this layer are cen- 

 tripetal, and are the direct continuations of the axones of the cells of the next layer, but a few of 

 the axones are centrifugal, being axones of ganglion cells within the brain. The centrifugal 

 fibres in the layer of nerve fibres pass through it and the next succeeding layer to ramify in the 

 inner plexiform and inner nuclear layers, where they terminate in enlarged extremities. The 

 layer of nerve fibres is thickest at the optic nerve entrance, and gradually diminishes in thickness 

 toward the ora serrata. 



3. The ganglionic layer consists of a single layer of large ganglion cells, except in the macula 

 lutea, where there are several strata. The cells are somewhat flask-shaped, the rounded internal 



