48 THE EXTERIOR OF THE HORSE. 



D. The Eye (Fig. 20 and Fig. 23). 



Situation ; Limits. The eye constitutes a double region situ- 

 ated upon the lateral planes of the head, above the cheek and the face, 

 below the supra-orbit, and on each side of the forehead. 



Anatomical Base. The essential part of the visual apparatus 

 consists of a membranous sphere called the ocular globe, transparent 

 in front, whose interior is lined by a nervous membrane, the retina, 

 sensitive only to luminous rays. 



It is protected over its greater extent by a deep osseous cavity, the 

 orbital cavity, to the bottom of which it is fixed; in front also by 

 two mobile curtains, the eyelids, which complete the orbit in front and 

 regulatp the quantity of light which should be admitted into its inte- 

 rior. Ten pairs of muscles endow it with various movements ; finally, 

 special organs of a glandular nature lubricate its anterior surface and 

 protect it from the constantly desiccating influence of the atmosphere. 

 Such, in a general way, is the organization of the ocular region. 



1st. Essential Organs of Vision, or the Globe of the Bye. This is 

 a membranous envelope, completely closed, whose interior is filled with trans- 

 parent substances of a variable density, known under the generic name of the 

 media of the eye. 



This sac, most bulging in front, is constituted here by a transparent, thick, 

 and resisting expansion, the lucid cornea, covering one-fifth of its peripheric sur- 

 face. Posteriorly it is composed of three layers, to which are assigned different 

 functions. These are, proceeding from without to within : 



a. The sclerotic (1), white, sometimes pigmented, 'fibrous, resisting, and 

 thick, which is vulgarly often known under the name of the white of the eye, 

 and which is visible from the exterior by its white coloration around the periph- 

 ery of the cornea whenever the eyelids are separated from each other. Its ex- 

 ternal surface gives attachment to the motor muscles of the eye-globe. Its 

 anterior elliptical opening is closed by the cornea by bevelled edges after the 

 manner of the crystal fitting into the case of a watch. 



b. The choroid (2), very thin, black, not visible from the exterior, which 

 plays the role of an absorbing surface for an excess of luminous rays, and con- 

 verts the interior of the eye into a veritable dark chamber, into which the 

 images of external objects are received to be appreciated by the retina. 



This membrane, in front, where it is called the ciliary body, presents numer- 

 ous radiating folds, the ciliary processes, which circumscribe the edges of the crys- 

 talline lens (7). Through the ciliary ligament it gives attachment to the circum- 

 ference of the iris (4), an elliptical diaphragm pierced in its centre by an opening, 

 the pupil (5). 



The iris and the pupil are visible through the transparent cornea. The 

 former divides the space anterior to the crystalline lens anterior chamber into 

 two communicating compartments, an anterior (8) and a posterior (8') practically 

 obliterated. The coloration of its anterior face is of a yellowish-brown, but it may 

 vary in different subjects ; that of the posterior, on the contrary, is black, due to 



