THE FIELD OF VISION 



29 



duce an extension of the monocular field of vision in the 

 horizontal plane. 



In the center of these elongated oval pupils a deeply- 

 pigmented elevation is met with at both the upper and 

 lower borders, but considerably larger and more raised at 

 the upper than the lower (Fig. 5). This elevation has 

 received the name of the "corpus nigrum." In a horse, 



A- 



FIG. 4. Sections through the iris of a horse. .4, in the horizontal meridian. 



B, in the vertical meridian. 



the lateral diameter of whose pupil measured 17 mm., the 

 corpus nigrum at its upper border measured 8 mm. in length, 

 3 mm. in width and 3.5 mm. in height. In sections of the 

 iris it can be seen to be an extension forward from the 

 pigment epithelium on its posterior surface, i. e., a growth 

 forward from the anterior extremity of the secondary optic 

 vesicle (Fig. 4 B). It is a vascularized structure and so 

 has some elements of mesoblastic tissue in it. Bleached 



