808 



THE OBGANS OF SENSE. 



Ligamentum pectinatum iridis 

 Scleralspur [ Radial muscle of iris 



Venous sinus of sclera i e T 



Sclera 



sclerse. It is thinner in the child than in the adult, and presents a bluish appearance 

 caused by the pigment of the chorioid shining through it ; in old age it assumes 

 a yellowish tinge. In front of the equator it gives attachment to the tendons of 

 the ocular muscles, while its anterior part is covered by the conjunctiva. Its deep 

 surface presents a brownish colour, and is loosely attached to the chorioid, except 

 at the entrance of the optic nerve and in the neighbourhood of the sulcus sclerae. 

 It is pierced, behind, by the optic nerve, the entrance for which is funnel-shaped, 

 wide behind and narrow in front, and is situated 3 mm. to the nasal side and 

 slightly below the level of the posterior pole. The fibrous sheath of the nerve blends 

 with the outer part of the sclera, while the nerve bundles pass through a series of 

 orifices ; this perforated portion is named the lamina cribrosa sclerae. Around the 

 entrance of the optic nerve are some fifteen to twenty small apertures for the 

 passage of the ciliary nerves and short ciliary arteries. The two long posterior 

 ciliary arteries pierce it, one on each side, some little distance from the entrance of 

 the optic nerve ; while a little behind the equator are four openings, two above and 

 two below, for the exit of veins, called venae vorticosae ; near the sulcus sclerse it is 

 perforated by the anterior ciliary arteries. The deep surface of the sclera is lined 



with flattened endothelial 

 cells; and between it and 

 the chorioid is an exten- 

 sive lymph space, the 

 spatium perichorioideale, 

 which is traversed by the 

 ciliary nerves and arteries 

 just mentioned, and by 

 an irregular mesh work of 

 fine, pigmen ted, connective 

 tissue, the lamina fusca, 

 which loosely attaches the 

 sclera to the chorioid. At 

 the sclero-corneal junction 

 the fibrous tissue of the 

 sclera passes continuously 

 into that of the cornea, and 

 in the deeper part of this 

 \ Parts of ciliary processes junction there is a circular 



Circular fibres of ciliary muscle ^^ the ginus venogus 

 FIG. 678. SECTION OF IRIDIAL ANGLE. (Prof. Arthur Thomson.) sclerae (O.T. canal of 



Schlemm) (Fig. 678). 



When seen in a meridional section of the sclero-corneal junction, the sinus 

 venosus sclerse appears as a narrow cleft ; its outer wall is formed by the compact 

 tissue of the sclera, while its inner consists of a triangular mass of trabecular tissue ; 

 the apex of the triangle is directed forwards and is continuous with the posterior 

 elastic lamina of the cornea. The sinus is lined with endothelium, and 

 occasionally contains a few red blood corpuscles. It communicates, on the one 

 hand, with the anterior ciliary veins, and on the other, through the spatia anguli 

 iridis in the trabecular tissue, with the anterior chamber of the eye. 



Structure. The sclera consists of bundles of white fibrous tissue, together with some fine 

 elastic fibres, the bundles forming equatorial and meridional layers, which interlace with each 

 other. Numerous spaces containing connective tissue cells and migratory cells exist between the 

 fibres. Pigmented cells are plentiful in the lamina fusca, and a few are found also in the tissue 

 of the sclera, near the entrance of the optic nerve, and in the region of the sclero-corneal junction. 



Vascular and Nervous Supply. The sclera receives its blood-supply from the short 

 posterior ciliary and the anterior ciliary arteries, while its veins open into the venae vorticosae and 

 anterior ciliary veins. The cell spaces play the part of lymph vessels and communicate with 

 the pericborioidal and suprascleral lymph spaces. Its nerves are derived from the ciliary 

 nerves, which, after losing their medullary sheaths, pass between the fibrous bundles ; their exact 

 mode of ending is not known. 



Cornea. The cornea is transparent and forms the anterior sixth of the outer 

 tunic; its index of refraction is from 1'33 to 1/35 ; the thickness of its central part 



Meridional fibres of ciliary muscle 



Iridial angle 



