340 ZONULA CILIABIS CORNEA. 



centralis, the central artery of the retina,, which pierces the 

 optic nerve, and then, by its expansion, forms the principal 

 support for the nervous layer. The nervous layer is de- 

 scribed by Treviranus, as consisting of cylindrical tubes, or 

 fibres extending, in every direction, from the optic nerve, 

 and terminating in papilla? which are in contact with the 

 hyaloid membrane. 



The external or serous layer separates the retina from 

 the pigmentary coat of the choroid, and when suspended 

 in water, is seen as an extremely delicate membrane, which 

 under the microscope presents granules having a tesselated 

 arrangement. On the inner surface of the retina, at the 

 bottom of the eye, where the optic nerve enters, a dark 

 point is seen, called the porus options ; through this the 

 arteria centralis passes, and then divides. 



From this point, about a line and a half to Jhe outside, 

 is seen a small, circular spot,, having a yellowish border, 

 called the foramen of Soemerring, or punctum aureum. This 

 spot corresponds to the axis of the eyes, and is said not to 

 exist in the foetus. It is found in man and the quadru- 

 mana that is, in all animals the axes of whose eyes are 

 parallel. It is not seen in the horse, the ox, nor other 

 mammalia, in birds nor in fish. This spot is not a fora- 

 men, as supposed by Soemerring, but is found to be covered 

 both by the vascular layer, and the serous layer of Jacobs, 

 there being a depression, and an apparent deficiency of the 

 nervous substance at this point. 



Zonula Ciliaris. This is a term applied to the thin vas- 

 cular structure connecting the anterior surface of the lens, 

 with the anterior margin of the retina. It is found to 

 present folds, corresponding with, and received between 

 the ciliary processes, arranged in a radiated form, and 

 stained with the pigment around the lens. The membra- 

 nous case of the ball of the eye thus formed of its three 

 coats, the sclerotic, choroid, and retina, contains four 

 refractive bodies, or media of light: the cornea, aqueous 

 humor, crystalline lens, and vitreous humor. 



Corneaj (corneus, horny,) Fig. 98. This structure occu- 



