740 THE SENSE ORGANS AND SKIN OF THE HORSE 



It has the form approximately of an oblate spheroid, Init is composed of the 

 segments of two spheres of different sizes. The anterior, transparent, corneal 

 segment has a radius of curvature of about 17 mm., and the posterior, opaque, 

 scleral segment a radius of about 25 mm. The anterior segment therefore projects 

 more strongly, and the junction of the two segments is marked externally by a 

 broad, shallow groove, the sulcus sclerae. The central points of the anterior and 

 posterior curvatures of the eyeball are termed respectively the anterior and pos- 

 terior poles (Polus anterior, posterior), and the line connecting the poles is the optic 

 axis (Axis oculi externa). The angle of divergence of the optic axes is about 

 137 degrees. The equator (^Equator) is an imaginary line drawn around the eye- 

 ball midway between its poles, and meridians (Meridiani) are lines drawn around it 

 through the poles. 



The average transverse diameter of the eyeball is about 5 cm., the vertical about 4. .5 cm., 

 and the axial about 4.25 cm. The distance from the anterior pole to the point of entrance of the 

 optic nerve is about 3 cm. 



Lens seen through cornea 

 \ 



Corneoscleral junctiorL \ 



Third eyelid^ 



Caruncula lacriinalis. ^x m\ ^r^w^ x c , 



^ bclera 



"^Coryiea 



^Pupil 

 27 > 



Tig. 5.56.^Left Eykbai.l of Horse, in situ, after Removal of Upper and Lower Lids. 

 9 Zygomatic arch; 12, .supraorbital process; 19, orbital fat; 27, facial crest. (After Ellenberger-Baum, Anat. 



fur Kiinstler.) 



The eyeball consists of three concentric tunics or coats, within which three 

 refractive media are inclosed. 



The Fibrous Tunic 



The fibrous tunic (Tunica fibrosa oculi) is the external coat and is composed 

 of a posterior opaque part, the sclera, and a transparent anterior part, the cornea. 



The sclera, popularly termed the "white of the eye," is a dense fibrous mem- 

 brane which forms about four-fifths of the fibrous tunic. Thickest in the vicinity 

 of the posterior pole (ca. 2 mm.), it thins at the equator (ca. 0.4 mm.), and increases 

 in thickness toward the junction wdth the cornea (ca. 1.3 mm.). It is in general 

 white, but may have a bluish tinge in its thinnest parts. Its external surface 

 furnishes insertion to the ocular muscles and is covered by the conjunctiva sclera* 

 in its anterior part. The episcleral tissue, which is richly supplied with vessels 

 and nerves, attaches the conjunctiva to the sclera; it is abundant and loosely 

 meshed except at the junction with the cornea. The inner surface is attached to 

 the chorioid coat by a layer of delicate, pigmented, connective tissue, the lamina 



