14 OPHTHALMOLOGY FOR VETERINARIANS 



The coats of the posterior five-sixths of the globe are 

 three in number, and from without inward are called 

 the sclera, the chorioid, and the retina. The anterior 

 one-sixth is formed by the transparent cornea. 



The eye internally is composed of the anterior cham- 

 ber, the iris and cihary body, the posterior chamber, 

 the lens in its capsule suspended by the cihary liga- 

 ment from the ciHary muscle, the vitreous encased in 

 the hyaloid membrane, and the optic disk. 



The sclera is a white, tough, fibrous membrane, 

 and extends from the optic nerve to the cornea. It 

 is really an expansion of the dura mater, which ^tends 

 forward from the skull cavity through the optic foramen 

 in the apex of the orbit, and serves, in its course to the 

 eyeball, as a sheath for the optic nerve. It. is principally 

 for protection, and affords attachment for the extrinsic 

 muscles. The four recti and the two oblique muscles 

 have their attachment anterior to the equator, while 

 the retractor muscle is widely expanded over the poste- 

 rior third of the sclera, which is its thickest portion. 



The fine fibrillae of which the sclera is composed run 

 in two principal directions — from before backward, and 

 in a circular direction concentric with the corneal 

 margin. 



Anteriorly, the sclera is continuous with the true cor- 

 nea. At the posterior portion a few fibers of the inner 

 layer penetrate the optic nerve at a junction with the 

 trunk of the nerve and its head, and are inserted into 



