LATERAL FACES OF THE HEAD 49 



the presence of pigment called the " uvea," variable masses of which, called " cor- 

 pora nigra," or " soot-balls," • are sometimes suspended from its edge into the 

 pupillary opening. 



Fig. 20.— Vertical section of the eye of the horse (semi-schematic). 



1, sclerotic; 2, choroid; 3, ciliary processes; 4, iris; 5, anterior compartment, with the 

 aqueous humor; 6, retina; 7, crystalline lens; 8, anterior chamber ; 8', posterior chamber: 9, pos- 

 terior compartment of the eye, with the vitreous humor; 10, ocular sheath ; 11, tarsal cartilages; 

 12, integument of the eyelids; 13, conjunctiva; 14, section of the small oblique muscle; 15, lach- 

 rymal gland ; 16, cornea; 17, transverse section of the superior half of the orbicularis muscle; 

 18, posterior straight muscle; 19 superior straight muscle; 20, elevator of the superior eyelid; 

 21, section of the orbital arch ; 22, optic nerve ; 23, transverse section of the inferior moiety of the 

 orbicularis muscle ; 24, section of the floor of the orbit. 



c. The retina (6), transparent, very thin and delicate, adherent to the choroid 

 and its dependencies, which is an expansion of the optic nerve, and upon which 

 the impressions of light are directly perceived. 



Media of Refraction. — a. Crystalline Lens (7). — This is a bi-convex, 



1 These are found in most eyes, and when not excessive, so as to obstruct the light, are not 

 an unsoundness. (Harger.) 



4 



