(US MANUAL OF HISTOLOGY. 



Besides these parts, we have in the eye a complicated vascular system, 

 derived almost exclusively from the ophthalmic artery. This may be con- 

 sidered as consisting of several divisions, with separate afferent and efferent 

 vessels, but communicating one with another. These are (a) the system 

 of the retina, (b) the ciliary system, and (c), as far as the globe of the eye 

 is covered by the conjunctiva, the conjunctival system. 



REMARKS. (1.) Compare Bowman, Lectures on the parts concerned in the opera- 

 tions of the eye, &c. London, 1840. 



309. 



The sclerotic, the hard, or white tunic of the eyeball, belongs to the 

 large group of fibrous membranes. Like these it consists of a dense inter- 

 lacement of connective-tissue bundles, intermixed with numerous fine 

 elastic fibres, which appear in greatest number on the internal concave 

 surface. The mode in which these bundles are interwoven is peculiar 

 one set anastomosing around the entrance of the optic nerve, and radiating 

 from thence meridionally towards the edge of the cornea, while another 

 set is arranged parallel to the equator of the eyeball. Thus the fasciculi 

 intersect each other at right angles (Loewig). 



Close to the point of insertion of the cornea, the inner surface of the 

 sclerotic coat is traversed by a complicated circular sinus a regular plexus 

 of venous twigs (fig. 575, d) known as the canalis Schlemmii, to which 

 we shall be obliged to refer again in speaking of the vessels of the choroid. 

 Posteriorly, the outer portion of the sclera is connected directly by means 

 of its meridional fibres with the external covering of the optic nerve, 

 derived from the dura mater. The internal neurilemmatic substance of 

 the nerve, and the lamina cribrosa, and inner part of the sclerotic, also 

 merge one into the other. Anteriorly, the latter is strengthened by the 

 addition to its meridional layer of fibres of the tendons of the recti muscles, 

 while those of the obliqui unite with the equatorial fasciculi in the pos- 

 terior segment. As has been already mentioned, this hard membrane of 

 the eyeball is poor in vessels, its fine capillaries presenting rather large- 

 meshed networks (Briicke). These we shall be obliged to refer to again 

 in connection with the vascular system of the bulbus. Nerves are said 

 to have been met with in this structure in the rabbit (Rahm). 



The cornea (fig. 576, a), with its two transparent limiting membranes 

 (b, c), has been already minutely described ( 133). The laminated flat- 

 tened epithelium of its anterior surface (d), to which the name of con- 

 junctival layer of the cornea has been given, was also considered, and also 

 the simple cellular covering of the posterior surface (e), ( 87 and 88). 



The peculiar chondrin-yielding tissue of the cornea becoming somewhat 

 changed towards the periphery, is continuous with the ordinary collagenic 

 connective-tissue of the sclerotic, more particularly with its meridional 

 fasciculi. At its edges the membrane of Descemet undergoes a peculiar 

 transformation into streaky membranous masses, which are then disposed 

 in various ways. The most external pass partly into the posterior wall 

 of the canal of Schlemm, and are in part lost in the ciliary muscle of 

 the choroidea ; the internal break up eventually into bands and cords, 

 which pass across the anterior chamber, disappearing in the tissue of 

 the iris. They thus constitute the ligamentum pectinatum of the latter 

 (see below). 



In adults the cornea appears almost completely destitute of blood- 

 vessels. At the border alone a narrow row of them, from 1*1 to 2*3 mm. 



