EYE OF THE ENGLISH SPARROW 421 



hidden by the pecten (P) except a small portion where it enters. 

 The area centraHs (A) is a shghtly thickened elliptical region 

 with a funnel-like depression in the center which forms the fovea. 

 It is broader in the horizontal diameter than in the vertical, 

 being respectively 1 mm. by .8 mm. The area of this thickened 

 region, the area centralis, is 6.2832 sq. mm. The outline of the 

 depression forming the forea conforms in general to that of the 

 area centralis. This is seen in the enlarged drawing, fig. 5, 

 plate 1. The shape of the foveal depression can be made more 

 clear by consulting plate 2, figs. 6, 7, and 8. These figures are 

 camera-lucida outlines of horizontal and vertical sections through 

 the fovea at right angles to the surface of fig. 5. The horizontal 

 outline has a broader slope than the vertical The extreme 

 bottom of the fovea forms a much sharper bend in the vertical 

 sections than in the horizontal. This can also be observed in 

 the microphotographs of vertical sections through the fovea 

 (figs 24, 25, and 26, plate 5, as compared with the horizontal 

 sections in figs. 29, and 30 of' plate 6, 33 and 34 of plate 7, 

 and 43 and 44 of plate 8. 



The exact shape of this foveal depression at different levels is 

 readily observed in camera-lucida drawings of serial sections 

 made tangential to the retina in this region. This would be the 

 same as sections passing through figure 6 perpendicular to the 

 drawing and parallel to the line H-H. 



Figure 9 A, plate 2, represents a tangential section at the 

 extreme bottom of the foveal depression. Its horizontal diam- 

 eter is .016 mm. and its vertical, .008 mm. This has an area of 

 0.0001 sq. mm. The successive drawings of this figure represent 

 sections at different levels taken .015 mm. apart. The last one, 

 fig. 9, M, represents the shape at the inner margin of the retina 

 or at the beginning of the depression. It measures in a hori- 

 zontal direction .432 mm. and in a vertical, .300 mm. The 

 general shape of the foveal depression is that of a truncated, 

 elliptical funnel with trumpet-shaped wall, whose dmiensions 

 are .432 X .300 mm. at the mouth, .016 X .008 mm. at the 

 truncated end, and .180 mm. deep. The total area of the sur- 

 face of this depression is 0.0997 sq. mm. Microphotographs of 



