548 



THE NERVOUS SYSTEM. 



out the nervous elements present in the pars optica. This portion 

 passes forward as far as the outer margin of the iris. 



Macula Lutea (Figs. 326, 327). At the center of the retina 

 posteriorly, and at the posterior extremity of the axis of the eye, 



FIG. 328. Diagrammatic representation of the retina. 



is the macula lutea, yellow spot of Sommerring, or limbus luteus, 

 having a diameter of from 1 to 2 mm., an elevated spot in the 

 retina where the sense of sight is most acute ; in its center is a 

 depression, the fovea centralis, whose diameter is from 0.2 mm. 



FIG. 329. Schematic diagram of the retina, according to Ramon y Cajal: the 

 line a, after passing through a Mailer's fiber, crosses a bipolar rod-cell, then two 

 bipolar cone-cells, and finally ends in the body of a bipolar cone-cell, a', Nerve- 

 fiber layer ; 6, ganglion-cell layer ; c, inner molecular layer ; d, spongioblast ; 

 e, nucleus of Miiller's fiber ; /, inner nuclear layer ; g, bipolar cone-cell ; h, outer 

 molecular layer ; i, outer nuclear layer ; fc, cone ; Z, rod ; m, centrifugal nerve-fiber ; 

 n, impressions on Miiller's fiber of elements of outer nuclear layer ; o, fiber basket of 

 Miiller's fiber; p, diffuse spongioblast ; q, large horizontal cell ; r, bipolar cell. 



to 0.4 mm. At the fovea the retina is so thin that the color of 

 the choroid can be seen through it, and hence it has somewhat the 

 appearance of an opening, and was formerly called foramen of 

 Sommerring. For further discussion of the macula the reader is 

 referred to p. 553. 



