PHYSIOLOGY. 



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and covers the white part of the globe of the eye. This 

 membrane has numerous blood-vessels, which are conspicuous 

 when it is inflamed. The bulb or globe of the eye is com- 

 posed of several coats and humors. The transparent cornea, 

 which, in the horse, forms the front part of the eye, com- 

 prehends a larger part of the globe than in the human subject ; 

 on removing this cornea, a fluid, which is named the aqueous 

 humor, escapes, and the iris appears. The iris is a muscular 

 curtain, having a hole in the centre, which is termed the pupil. 

 This divides the fore part of the eye into two parts, named 

 chambers, which are occupied by the aqueous humor. The 

 pupil is of a dark bluish cast ; is of an oval, or rather of an 

 oblong form. The iris regulates the quantity of light that is 

 required to pass through the pupil. For this purpose, it is 

 composed of two sets of muscular fibres ; by means of one 

 the pupil is enlarged, and by the other it is diminished. Thus, 

 if the pupil is first examined in the stable, where there is a 

 moderate light, and immediately after in the sunshine, it will 

 be found quite altered ; being so small, in a strong light, as to 

 be nearly closed. On removing the iris, the second humor, 

 or crystalline lens, appears : this is retained in its situation by 

 a transparent membrane, named its capsule, between which 

 and the lens is a minute quantity of fluid. The third humor 

 of the eye is the vitreous. This humor is not contained in 

 one general sac, but in numerous minute and perfectly trans- 

 parent cells, and resembles pure water : this humor serves to 

 produce a small degree of refraction in the rays of light, and 

 occupies and distends all the posterior part of the globe of the 

 eye. The next coat to the conjunctive is the sclerotica, 

 or white of the eye, a strong, thick membrane, which extends 

 from the transparent cornea to the optic nerve. The next 

 coat to the sclerotic is the choroid. This is a delicate and 

 very vascular membrane. In the human eye, it appears of 

 a black color, and it is this which causes the pupil of the 

 human eye to appear black ; but the choroid coat of the 

 horse's eye is variegated in color ; in some parts black, in 

 others blue ; and in others green. The next coat is the 

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