SENSE OF SIGHT. 181 



sponding segment of the retina when the interior of the eye is 

 examined. These are due to imperfect closure of the choroidal 

 fissure. The choroidal fissure is the result of the peculiar mode 

 in which the optic vesicle is cupped or invaginated. The lens 

 grows into it from the malar or lower lateral aspect. The lens is 

 lodged in the anterior part of the depression ; the posterior part 

 becomes the choroidal fissure (Fig. 147). The margins of the 

 fissure unite, all traces of it normally disappearing. The margin 

 of the cup left after the union of the lips of the choroidal fissure 

 becomes the boundary of the pupil (Fig. 14*7). 



Binocular Vision. — At first the optic vesicles are directed 

 laterally in the human embryo, and in mammals generally the 

 eyes are so directed, each eye having its own field of vision. 

 In the Primates the eyes swing forwards during the second month; 

 binocular vision is thus made possible. With binocular vision 

 and the combination of images appear in the highest primates : — 



(1) A fovea centralis and macula lutea (L. Johnston) ; 



(2) A partial crossing of the optic fibres at the chiasma ; 



(3) Certain alterations in the attachments of the oblique 

 muscles of the eyeball. 



The cavity of the Optic Vesicle (Fig. 146) is of some clinical 

 importance. It is obliterated by the invagination of the vesicle; 

 the rods and cones formed in the inner or invaginated layer grow 

 out across the cavity into the outer or ensheathing pigmented 

 layer of the retina. From accident or disease the retina may be 

 detached; the separation takes place between the pigmented 

 epithelium, which remains in situ, and the rods and cones, which 

 fall inwards with the nerve layer. Fluid then collects in the site 

 of the primitive cavity of the optic vesicle. 



3. Parts of the Eyeball formed from the Mesoblast. — 



As the lens invaginates the optic vesicle and forms the optic 

 cup, it carries in with it the surrounding mesoblast. Thus 

 the lens is surrounded and the cup filled by mesoblast (Fig. 

 148). 



The structures formed from the mesoblast are : 

 ' (1) The Capsule of the lens. — It is developed out of the meso- 

 blast which surrounds the lens. At first the capsule is continuous 



