646 THE EYE 



According to certain investigators (Szily, 1908, et al.) the definitive 

 vitreous humor is preceded by a primitive structure in the form of a loose- 

 meshed reticulum of protoplasmic fibers (cytodesmata) derived from, and 

 in continuity with, both the lens and the retina, 

 hence ectodermal in origin. This primitive vi- 

 treous body is subsequently invaded by vascular 

 mesenchyma, the two tissues uniting to produce 

 the definitive vitreous. The hyaloid membrane, 

 surrounding the vitreous body, and the fibers of 

 the suspensory ligament of the lens are thought 

 by some to represent the persistent peripheral 

 FIG. 545. SCHEMATIC RE- remains of the original ectodermal stroma. 

 CONSTRUCTION OF THE Others, among them most recently Baldwin 

 DEVELOPING EYE. (Arch. mikr. Anat., vol. 80, 1912), describe the 



a,opticcup; S ,choroidal ori ^ in of the suspensory ligament from mesen- 

 fissure; s,, optic nerve; L, chyme. 



developing lens. (After Failure of closure on the part of the chor- 



Fuchs.) oidal fissure gives rise to various degrees of a 



defect known as coloboma. This condition may 



appear simply as a shallow cleft in the iris, or as a more or less extensive 

 fissure in the retina or even the optic nerve, which seriously interferes with 

 vision. 



LAYERS OF THE RETINA 



The retina may be said to consist of ten layers, which from without 

 inward are : 



1. The pigment epithelium. 



2. The layer of rods and cones. 



3. The external limiting membrane. 



4. The outer nuclear layer. 



5. The fiber layer of Henle. 



6. The outer reticular layer. 



7. The inner nuclear layer. 



8. The inner reticular layer. 



9. The ganglion cell layer. 

 10. The nerve fiber layer. 



To these several layers an additional one, the internal limiting mem- 

 brane, is frequently added. The first five of these layers are contained 

 within the neuro-epithelial portion of the retina, the last five form its 

 cerebral or neural portion. 



1. The pigment epithelium (layer of pigmented cells) consists of 



