ON THE ESSENTIAL ORGANS OF SIGHT. 1^7 



c d- 3- 



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Pig. 538.— Theoretical Section of the Horse's Eye. 



a.— Optic nerve. 6.— Sclerotic, c— Choroid.— rf.—Ketina. «— Cornea. /.— Iris.— 

 ^^.— Ciliary circle (or ligament) and processes given off by the choroid, though repre- 

 sented as isolated from It, in order to indicate their limits more clearly. 1— Insertior 

 of the ciliary processes on the crystalline lens, j.— Crystalline lens, fc— Crystalline 

 capsule. Z.— Vitreous body, ww.— Anterior and posterior chambers, o.— Theoretical 

 indication of the membrane of the aqueous humor. ^'P-— Tarsi. (77— Fibrous m«m- 

 brane of the eyelids, r.— Elevator muscle of the upper eyelid, s s.— Orbicularis muscle 

 of the eyelids. <.— Skin of the eyelids, u —Conjunctiva. ■;;.— Epidermic layer of this 

 membrane covering the cornea, a;.— Posterior rectus muscle, jr.— Superior rectus 

 muscle, z— Inferior rectus muscle, w -Fibrous sheath of the orbit (or orbital mem- 

 brane). 



To this essential organ are added as accessories, first, a mus- 

 ■cular apparatus, constituted by seven muscles — di posterior straight 

 or retractor, four others, also straight, the superior, inferior, ex- 

 ternal and internal; and two oblique, or rotators, the great and 

 small, or external and internal oblique; second, an adipose pad; 

 thu'd, an apparatus of lubrication, composed of the lachrymal 

 gland and its means of conducting the tears, the product of its 

 secretion, viz., the hygrophthahnic canals, the puncta lachrymalis, 

 the caruncula lachrymalis, the lachrymal ducts, the lachrymal 

 sac, and the lachrymal canal. 



The whole mass of this apparatus is enclosed in a conical 

 fibrous sac, the ocidar sheath, which forms a membranous lining, 

 as it were, to the orbital cavity, or bony box, which is anteriorly 

 oj^en, except when closed by the eyelids. 



Our design in the present chapter is to confine our considera- 

 tion entirely to such portions of surgical ophthalmology as are 

 likely to demand the careful and practical attention of the vet- 

 erinarian. 



