THE FIBROUS TUNIC 



863 



Lens seen through cornea 

 Corneoscleral \ 



Third 

 eyelid 



Caruncula 

 lacrimalis 



posterior opaque, scleral segment one 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 posterior poles (Polus anterior, posterior), and the line connecting the 

 poles IS the external optic axis (Axis oculi externa). i The angle of divergence of the 

 optic axes is about 137 degrees. The equator (Equator) is an imaginary line drawn 

 around the eyeball 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. 



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

 refractive media are enclosed. 



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. 



1. The sclera^ is a dense fibrous 

 membrane 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 thick- 

 ness toward the junction with the 

 cornea (ca. 1.3 mm.). It is in gen- 

 eral white, but may have a bluish 

 tinge in its thinnest parts. Its ex- 

 ternal surface furnishes insertion to 

 the ocular muscles and is covered by 

 the conjunctiva sclerae 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 junc- 

 tion with the cornea. The inner 

 surface is attached to the chorioid 

 coat by a layer of delicate, pig- 

 mented connective tissue, the 

 lamina fusca. The anterior bor- 

 der, which is oval, the long axis 



being transverse, is continuous with the cornea. The transition from the opaque 

 scleral tissue to the transparent corneal substance occurs in such manner that the 

 sclera appears to form a groove (Rima cornealis), into which the cornea fits some- 

 what as a watch-glass in the case. Near the corneo-scleral junction there is a 

 circular venous plexus, the' plexus s. sinus venosus sclerae.^ The optic nerve 

 passes through the posterior part of the sclera a little below and lateral to the 

 posterior pole. The opening for the nerve is crossed by interlacing fibrous strands, 

 forming the lamina cribrosa sclerae. The sclera consists of interlacing bundles of 

 white fibrous tissue, associated with which there are a few elastic fibers. The 



1 The term internal optic axis (Axis oculi externa) is apphed to a coincident line from the 

 posterior surface of the cornea to the anterior surface of the retina. 



^ Also termed the sclerotic coat, and popularly known as the "white of the eye." 

 ' Formerly termed the canal of Schlemm. 



Sclera 



' Cornea 



Fig. 693. — Left Eyeball of Horse, in situ. After Removal 

 OF Upper and Lower Lids. 

 9, Zygomatic arch; IS, supraorbital process; 19, orbital 

 fat; 27, facial crest. (After EUenberger-Baum, Anat. fiir 

 Kiinstler.) 



