THE INTERNAL COAT THE NERVOUS TUNIC 



645 



visual axis. The forea 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, the physiologic excavation, 

 is placed 3.5 to 4 mm. to the nasal side of the macula lutea, and at a 

 slightly lower horizontal plane. 



DEVELOPMENT OF THE EYE 



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 re- 

 garded as a detached lobe of the cerebrum itself. The evagination or 

 optic vesicle grows forward 

 in the embryo, and soon 

 forms a flask-shaped process 

 whose expanded extremity 

 is early infolded in a cup- 

 like manner forming the 

 optic cup. The peripheral 

 layer of the cup becomes 

 the pigmented layer, the 

 lining layer differentiates 

 into the neural layers, of 

 the retina. The inferior 

 surface of this optic cup 

 and the connecting optic 

 stalk at first presents a 

 s 1 i t - 1 i k e deficiency, the 

 choroidal fissure, into which 

 grows the mesoblastic tis- 

 sue 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 developing 

 lens, uliich arises, under the influence of the optic cup itself (Lewis, 

 Amer. Jour, of Anat., 1904), but is formed from the overlying area of 

 the epidermal ectoderm. 



DEVELOPING EYE IN MERID- 

 IONAL SECTION; DIAGRAMMATIC. 



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

 lens; M, M, mesoblast; a, optic vesicle, pro- 

 truding from, 6, the first cerebral vesicle; c, a 

 thickening of the ectoderm, 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, anlage 

 of the neural portion of the retina. (After 

 Fuchs.) 



