THE EYE. 



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posterior part of the retina, corresponding to the axis of the eye, and at a point 

 in which the sense of vision is most perfect, is an oval yellowish spot, called, after 

 its discoverer, the yellow spot or macula lutea of Sommerring, having a central 

 depression, thefovea centralis. The retina in the situation of the fovea centralis 

 is exceedingly thin, and the dark color of the choroid is distinctly seen through 

 it ; so that it presents more the appearance of a foramen, and hence the name 

 " foramen of Sommerring " at first given to it. It exists only in man, the quad- 

 rumana, and some saurian reptiles. About one-eighth of an inch (3 mm.) to the 

 inner side of the yellow spot is the point of entrance of the optic nerve (porus 

 options) ; here the nervous substance is slightly raised so as to form an eminence 

 (colliculus nervi optict) ; 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." 



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

 microscopically by means of sections made perpendicularly to its surface, is found 

 to consist of ten layers, which are named from within outward, as follows : 



1. Membrana limitans interna. 



2. Layer of nerve-fibres (stratum opticum). 



3. Ganglionic layer, consisting of nerve-cells. 



4. Inner molecular, or plexiform, layer. 



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



6. Outer molecular, or 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 membrana 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 framework 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 through all the other layers of the retina, except the membrana 



FIG. 445.-The arteria centralis retina, yellow spot, etc., the anterior half of the eyeball being removed. 



(Enlarged.) 



limitans interna, to reach its destination. As the nerve passes through the lamina 

 cribrosa of the sclerotic coat, the fibres of which it is composed lose their medu 

 sheaths and are continued onward, through the choroid and retina, as simple axi 

 cylinders. When these non-medullated fibres reach the internal surface 



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