50 THE EXTERIOR OF THE HORSE. 



transparent l organ, more convex on its posterior face, and appears behind the 

 pupil. It is closely applied against the posterior face of the iris, and divides the 

 interior of the eye into two great compartments, an anterior (5) and a posterior (9). 



b. Vitreous Humor (9). It is a transparent, limpid, and gelatinous sub- 

 stance which occupies the whole of the posterior chamber. 



c. Aqueous Humor (8). The aqueous humor, slightly more dense 

 than water, occupies the anterior chamber (8 and 8 X ) of the eye. It bathes both 

 surfaces of the iris. It is under external as well as internal pressure, and readily 

 escapes when the cornea is perforated. 



The function of these three media of the eye is to concentrate the rays of 

 light, by refraction, upon the surface of the retina, where they make an image. 

 The lens, being more convex behind than in front, converges them for this 

 reason and brings them to a focus on the anterior surface of the retina. Its 

 relative position is thus necessitated by this function ; if too distant from the 

 retina, the image is formed anterior to the latter and is not perceived by that 

 membrane ; if too close, the rays converge posterior to the retina and the image 

 really would tend to be formed outside of the ocular globe and again is not 

 appreciated by this structure. 



2d. Protective Organs of the Eye-Globe. These include the orbital 

 cavity, the eyelids, and the nictitans membrane. 



a. The orbital cavity is a conical, deep cavity, surrounded by a fibro-osseous 

 wall with an osseous opening in front. Its parietes are formed by the ocular 

 sheath (10), conical and resisting, attached behind to the crest of the orbital hiatus, 

 whose base is attached to the orbital opening and thence prolonged into the 

 eyelids, whose base they constitute. Superiorly, the eye is only protected by the 

 adipose cushion which forms the base of the supra-orbital region. Internally 

 and inferiorly, the orbital cavity is osseous. The opening of the orbital cavity 

 is circumscribed by the orbital process of the frontal bone (21) and a portion of 

 the lachrymal and zygomatic bones (24). We also find on its floor and towards 

 the internal side the lachrymal fossa and the superior orifice of the canal of the 

 same name. 



6. The eyelids, distinguished as superior and inferior , are two mobile mus- 

 culo-membranous valves which protect the part of the eyeball exposed to the 

 exterior. Their free border, more curved in the superior than in the inferior, 

 presents the excretory orifices of the Meibomian glands as well as a series of ten- 

 tacular hairs longer above than below, known under the name of eyelashes. The 

 eyelids offer, besides, two commissures or angles, a temporal or external, and a 

 nasal or internal. 



The external face is covered by an adherent (12) delicate skin provided with 

 numerous short hairs. The internal face moulded on the eye-globe is covered by 

 a delicate mucous membrane the conjunctiva (13) which is very sensitive to 

 foreign bodies. The conjunctiva in the healthy animal is of a rosy color and 

 becomes reflected over the choroid and the nictitans membrane, and is pro- 

 longed into the lachrymal canal. It becomes arrested at the edge of the cornea 

 and its epithelial layer alone covers that structure. At the free border of the 

 eyelids exists a cartilaginous lamina called the tarsus, on which the extremity of 

 the ocular sheath terminates. 



1 It sometimes has a bluish color, due to the reflection of the variegated colors at the poste- 

 rior part of the choroid. (Harger.) 



