ANATOMY OF THE RETINA. 681 



terior portion of the eyeball, to a distance of about ^ of an. inch (1/7 mm.) 

 behind the ciliary processes. When torn from its anterior attachment, it 

 presents a finely serrated edge, called the ora serrata. This edge adheres 

 very closely, by mutual interlacement of fibres, to the zone of Zinn. In the 

 middle of the membrane, its thickness is about T ^ 7 of an inch (200 /*). It 

 becomes thinner nearer the anterior margin, where it measures only about -g-^ 

 of an inch (80 /A). Its external surface is in contact with the choroid, and 

 its internal, with the hyaloid membrane of the vitreous humor. 



The optic nerve penetrates the retina about ^ of an inch (3'2 mm.) within 

 and -^ of an inch (2-1 mm.) below the antero-posterior axis of the globe, 

 presenting at this point a small, rounded elevation upon the internal sur- 

 face of the membrane, perforated in its centre for the passage of the central 

 artery of the retina. At a point ^ to ^ of an inch (2*1 to 3'2 mm.) external 

 to the point of penetration of the nerve, is an elliptic spot, its long diameter 

 being horizontal, about ^ of an inch (2'1 mm.) long and -fa of an inch (0 - 7 

 mm.) broad, called the yellow spot of Sommerring, or the macula lutea. In 

 the centre of this spot, is a depression, called the fovea centralis. This de- 

 pression is exactly in the axis of distinct vision. The yellow spot exists only 

 in man and the quadrumana. 



The structures in the retina which present the greatest physiological im- 

 portance are the external layer, formed of rods and cones, the layer of nerve- 

 cells, and the filaments which connect the rods and cones with the cells. 

 These are the only anatomical elements of the retina, as far as is known, 

 except the pigment cells, that are directly concerned in the reception of 

 optical impressions, and they will be described rather minutely, while the 

 intermediate layers will be considered more briefly. 



Most anatomists recognize nine layers in the retina : 



1. Layer of pigment-cells (already described in connection with the 

 choroid). 



2. Jacob's membrane, the bacillar membrane, or the layer of rods and 

 cones. 



3. The external granule-layer. 



4. The inter-granule layer (cone-fibre plexus of Hulke). 



5. The internal granule-layer. 



6. The granular layer. 



7. The layer of nerve-cells (ganglion-layer). 



8. The expansion of the fibres of the optic nerve. 



9. The limitary membrane. 



The layer of rods and cones is composed of rods, or cylinders, extending 

 through its entire thickness, closely packed, and giving to the external sur- 

 face a regular, mosaic appearance ; and between these, are a greater or less 

 number of flask-shaped bodies, the cones. This layer is about ^-5- of an inch 

 (76 p.) in thickness at the middle of the retina ; yj-j- of an inch (62 /t), about 

 midway between the centre and the periphery; and near the periphery, 

 about y^-g- of an inch (55 /*). At the macula lutea the rods are wanting, and 

 the layer is composed entirely of cones, which are here very much elongated. 



