THE EYE. 349 



those of the connective tissue by being larger and more irregu- 

 lar in shape. 



Blood-vessels are found not only in the centre of the nerve, 

 but also scattered through various parts of the connective tis- 

 sues. 



At the lamina cribrosa there is an anastomosis with the ves- 

 sels of the circle of Holler, which, coming from the short poste- 

 rior ciliary arteries, forms a vascular circle in the sclera, about 

 the entrance of the optic nerve. 



Where the nerve-fibres pass through the sieve-like openings 

 of the lamina cribrosa they lose their medullary sheath, and 

 from that point pass on to the nerve-fibre layer of the retina 

 as transparent axis- cylinders ; but in rare cases the sheaths are 

 continued from the optic disk some little distance over the 

 retina, and are seen with the ophthalmoscope as very white 

 patches radiating out from the disk, or following the vessels 

 and gradually fading into the general color of the f undus by 

 a fine, fringe-like border. 



The vitreous body is a transparent, jelly-like mass, of spher- 

 ical shape, with a depression at the anterior part, in which the 

 lens rests. It is bounded behind and on the side by the retina, 

 in front by the lens with its attachments, and appears to have 

 no true hyaloid limiting-membrane of its own. It is very diffi- 

 cult to demonstrate any definite structure in this substance ; 

 toward the periphery it appears to be arranged somewhat in 

 concentric layers, but in the centre is more homogeneous. 



From the optic disk to the lens there is a small canal about 

 1 mm. wide in front and spreading out behind ; it is lined 

 with very transparent cells, and filled with a substance more 

 fluid than the rest of the vitreous ; it marks the position of the 

 arteria hyaloidea^ which is usually obliterated at about the 

 seventh foetal month. 



The vitreous body also contains numerous corpuscles, espe- 

 cially near the periphery ; these consist of round lymph- cells, 

 stellate cells, with one or more nuclei, and irregular arms, and 

 of branching cells which seem to have a transparent vesicle 

 filling up a part of their interior. The vitreous contains no 

 nerves, and after birth no blood-vessels ; it may be examined 

 fresh or hardened in a \ per cent, solution of chromic acid. 

 Sections may be colored blue with aniline, and preserved in 

 glycerine. 



