TUNICS OF THE EYE. 825 



is opaque, and formed by the sclerotic, the tunic of protection to the eyeball ; the 

 smaller sphere is transparent, and formed by the cornea. The term anterior pole is 

 applied to the central point of the anterior curvature of the eyeball, and that of 

 posterior pole to the central point of its posterior curvature; a line joining the two 

 poles forms its sagittal axis. The axes of the eyeballs are nearly parallel, and 

 therefore do not correspond to the axes of the orbits, which are directed outward. 

 The optic nerves follow the direction of the axes of the orbits, and are therefore 

 not parallel ; each enters its eyeball about 1 mm. below and 3 mm. to the inner or 

 nasal side of the posterior pole. The eyeball measures rather more in its transverse 

 and antero-posterior diameters than in its vertical diameter, the former amounting 

 to nearly an inch, the latter to about nine- tenths of an inch. 



The eyeball is composed of three investing tunics and of three refracting media. 



TUNICS OF THE EYE. 



From without inward the three tunics are : 



1. Sclerotic and Cornea. 



2. Choroid, Ciliary Body, and Iris. 



3. Retina. 



I. The Sclerotic and Cornea. 



The sclerotic and cornea (Fig. 440) form the external tunic of the eyeball ; they 

 are essentially fibrous in structure, the sclerotic being opaque, and forming the 

 posterior five-sixths of the globe ; the cornea, which forms the remaining sixth, 

 being transparent. 



The Sclerotic (axtypoz, hard) has received its name from its extreme, density 

 and hardness ; it is a firm, unyielding, fibrous membrane, serving to maintain the 

 form of the globe. It is much thicker behind than in front. Its external surface 

 is of a white color, and is in contact with the inner surface of the capsule of Tenon ; 

 it is quite smooth, except at the points where the Recti and Obliqui muscles are in- 

 serted into it, and its anterior part is covered by the conjunctival membrane: hence 

 the whiteness and brilliancy of the front of the eyeball. Its inner surface is 

 stained of a brown color, marked by grooves, in which are lodged the ciliary nerves 

 and vessels ; this is loosely connected by an exceedingly fine cellular tissue (lamina 

 fusca) with the outer surface of the choroid, an extensive lymph-space (peri- 

 choroidal) intervening between the sclerotic and choroid. Behind it is pierced by 

 the optic nerve, and is continuous with its fibrous sheath, which is derived from the 

 dura mater. At the point where the optic nerve passes through the sclerotic, this 

 tunic forms a thin cribriform lamina (the lamina cribrosa) ; the minute orifices in 

 this region serve for the transmission of the nervous filaments, and the fibrous 

 septa dividing them from one another are continuous with the membranous processes 

 which separate the bundles of nerve-fibres. One of these openings, larger than the 

 rest, occupies the centre of the lamella ; it transmits the arteria centralis retinae to 

 the interior of the eyeball. Around the cribriform lamella are numerous small 

 apertures for the transmission of the ciliary vessels and nerves, and about midway 

 between the margin of the cornea and the entrance of the optic nerve are four or 

 five large apertures, for the transmission of veins (vena? vorticosa?). In front, the 

 fibrous tissue of the sclerotic is directly continuous with that of the cornea by direct 

 continuity of tissue, but the opaque sclerotic slightly overlaps the outer surface of 

 the transparent cornea. 



Structure. The sclerotic is formed of white fibrous tissue intermixed with fine 

 elastic fibres, and of flattened connective-tissue corpuscles, some of which are pig- 

 mented, contained in cell-spaces between the fibres. These fibres are aggregated 

 into bundles, which are arranged chiefly in a longitudinal direction. It yields 

 gelatin on boiling. Its vessels are not numerous, the capillaries being of small 

 size, uniting at long and wide intervals. Its nerves are derived from the ciliary 

 nerves, but their exact roode of ending is not known. 



