458 HISTOLOGY 



Numerous tense elastic fibers are found especially in the deeper layers, 

 where they form a fine network over the posterior elastic membrane. 



Within the cement substance, there is a system of branched canaliculi, 

 dilated in places to form oval spaces. The latter are between the lamellae, 

 but the canaliculi extend also among the constituent fiber-bundles. 

 Within the spaces, there are flat stellate anastomosing cells or "corneal 

 corpuscles," the branches of which extend into the canals and tend to 

 unite with those of neighboring cells, at right angles (Fig. 466). The cells 

 and their processes are more or less surrounded by serous fluid. Leuco- 

 cytes enter the canals, and are normally found in the cornea; if the cornea 

 is inflamed they become abundant. Blood vessels and lymphatic vessels 

 are absent. 



The posterior basal or elastic membrane (Descemet's membrane) is a 

 structure clear as glass, 6 n thick. Its posterior surface is covered by a 

 simple layer of flat polygonal cells (Fig. 464), which form a part of the 

 lining of the anterior chamber. Toward the periphery of the cornea in 

 adults, the posterior surface of the elastic membrane presents rounded 

 elevations, and the posterior epithelium becomes continuous with the 

 anterior epithelium of the iris (Fig. 452). In this "angle," the cornea 

 receives connective tissue prolongations from the iris, which form the 

 pectinate ligament of the iris a structure highly developed in the horse 

 and cow, but rudimentary in man. 



BLOOD VESSELS. 



The central vessels of the retina supply a part of the optic nerve, and 

 the retina; the ciliary vessels supply the rest of the eye. These two sets 

 of vessels anastomose with one another only at the entrance of the optic 

 nerve (Fig. 467). 



The ciliary arteries include (i) the short posterior ciliary arteries; (2) 

 the long posterior ciliary arteries; and (3) the anterior ciliary arteries. The 

 three groups will be considered in turn. 



1. After supplying the posterior half of the surface of the sclera, some twenty 

 branches of the short posterior ciliary arteries penetrate the sclera around the optic 

 nerve. They form the capillaries of the lamina choriocapillaris. At the entrance of 

 the optic nerve they anastomose with branches of the central artery of the retina (c) 

 and thus form the circulus arteriosus nervi optici. At the ora serrata they anastomose 

 with recurrent branches of the long posterior ciliary and the anterior ciliary arteries. 



2. The two long posterior ciliary arteries also penetrate the sclera near the optic 

 nerve (Fig, 467, i). They pass, one on the nasal and the other on the temporal side 

 of the eye, between the chorioid and sclera to the ciliary body. There each artery 

 divides into two branches which follow the ciliary border of the iris, and connect with 

 the corresponding branches from the artery of the opposite side, thus encircling the 

 iris with an arterial ring. This is the circulus iridis major (Fig. 467, 2), from which 



