THE RETINA 



579 



life, with the exception of its pigment layer, is perfectly trans- 

 parent, but becomes opaque immediately after death or local injury. 

 The retina presents on its inner surface a slightly elevated yel- 

 low spot, the macula lutea (limbus luted], which lies exactly at the 

 posterior pole of the visual axis. The fovea centralis is the slight 

 depression in the center of the macula lutea, and is the result of an 

 apparent thinning of the retinal layers at this point. 



The papilla optica, or entrance of the optic nerve, also forming 

 a slight elevation with a central depression, is placed 0.5 to 1 mm. 

 to the inner side of the macula lutea, and at a slightly lower hori- 

 zontal plane. 



Development. A word as to the development of the organ will 

 make clearer the description of the several layers of the retina. The 



retina is developed as 

 an evagination of the 

 first cerebral vesicle, 

 and is, therefore, to 

 be regarded as a de- 

 tached lobe of the 



FIG. 423. THE DEVELOPING EYE IN TRANSECTION; 

 DIAGRAMMATIC. 



A, early ; B, later stage. E, E, ectoderm ; L, lens ; 

 M, M, mesoblust ; a, optic vesicle, protruding from, 6, 

 the first cerebral vesicle ; c, a thickening of the ecto- 

 derm, anlage of the lens ; o, constricted pedicle of the 

 optic cup ; p, outer coat of the optic vesicle, anlage of 

 the retinal epithelium; r, inner wall of the vesicle, an- 

 lage of the neural portion of the retina. (After Fuchs.) 



FIG. 424. SCHEMATIC RE- 

 CONSTRUCTION OF THE 

 DEVELOPING EYE. 

 a, optic cup ; s, fissure ; 

 i, optic nerve ; L, de- 

 veloping lens. (After 

 Fuchs.) 



cerebrum itself. The evagination grows forward in the embryo, 

 and soon forms a flask-shaped process whose expanded extremity 

 is early infolded in a cup-like manner. The inferior surface of 

 this optic cup at first presents a slit-like defi iency, into which 

 grows the mesoblastic tissue which ultimately forms the vitreous 

 humor and conveys the central artery of the optic nerve. The 

 indented extremity of the optic cup is soon occupied by the de- 

 veloping lens, which arises, under the influence of the optic cup 



