THE DISEASES OF CATTLE. 229 



junction of the optic nerve with the globe, to the cornea, and 

 more than any other tunic of the eye tends to preserve its 

 form. It also affords attachment for most of the muscles of 

 the eye, the retractor and four recti. 



Bd. Cornea. — This is a dense and compact membrane, 

 yet composed of laminated structure, which accounts for the 

 opacity or " filmy " obscurity so often encountered in veterinaiy 

 practice; for, in a physiological condition of the eye, the laminae 

 are compact, like a house newly shingled ; but, in consequence 

 of untoward circumstances, a leak occurs, a watery exudation 

 then sets in. The cornea may be considered as the shield that 

 protects the eye, yet in consequence of its transparency, it is 

 also operative in the production of vision. 



4:th. Iris. — This is the movable curtain of the eye, adapts 

 the eye to vision, possesses the property of contraction and ex- 

 pansion ; in its centre is a perforation commonly known as the 

 pupil. The dimension of the pupil depends entirely on its 

 conti-action and dilatation ; in a very dark barn (provided the 

 eye be in a healthy condition), the pupil will be very much di- 

 lated ; in a barn well lighted the pupil will be quite small. 

 The iris is a fibro-muscular membrane, abundantly supplied 

 with blood-vessels and nerves, and coated with a, substance 

 known as uvea which is derived from the pigment. The arte- 

 ries of the iris come from the ciliary, its nerves are derived 

 from the ciliary. 



5th. Choroid Coat. — This coat or membrane lies beneath 

 the sclerotica ; it extends from the optic foramen to the ciliaiy 

 edge of the cornea ; both of its surfaces are coated with pig- 

 mentj which absorb the rays of light ; the outer coating is en- 

 tirely black ; the inner one is a mixture of black and green, 

 which lines the posterior half of the dark chamber. 



Qth. The Retina. — This is the third or inner coat of the 

 eye ; it is a delicate, transparent membrane, highly vascular ; 

 the principal part of it, probably, consists of an expansion of 

 the optic nerve ; it is the part on which the object of vision is 

 depicted. It is supplied with blood by branches from the cen- 

 tral artery of the retina. 

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