412 CORNEA. 



the ciliary arteries and nerves enter. A little in front of these 

 are seen other orifices, by which the veins emerge from, the 

 interior ; and near its junction with the cornea, a few openings 

 are met with, which allow of the entry and exit of vessels, di- 

 rectly upon the iris. These latter vessels become very apparent 

 in iritis, but present a hue less intensely bright, than those more 

 superficial and more serpentine, which are involved in simple 

 conjunctivitis. * 



The Cornea. 



The transparent membrane which fills up the vacuity in the 

 anterior part of the sclerotica, is denominated Cornea, from its 

 resemblance to horn. It is said to be superior in strength to 

 the sclerotica ; and it is also very firm. It is formed of lamina, 

 that are separable from each other, which are supposed to be 

 connected by a very delicate cellular membrane. 



This cellular membrane appears sometimes to contain a fluid ; 

 for, if a section be made of the coats of the eye, and pressure 

 be applied to the cornea, an exudation will be perceived, both 

 upon its internal and external surfaces. 



The cornea is composed of six or seven distinct lamella, 

 laying over each other like the leaves of a book, but which 

 cannot very readily be separated by maceration. The cellular 

 tissue which connects them enables us to slide the layers 

 slightly over one another between the thumb and finger. The 

 cells of this tissue are filled with a fluid ; hence it has been 

 called by some writers, substantia spongiosa cornea. This 

 fluid performs the important office of preserving the trans- 



* The neurilema of the optic nerve which is an emanation from the dura 

 mater of the skull, is lost upon the sclerotic tunic. Hence, Galen, and many 

 writers since him, even Zinn and Meckel, consider the sclerotic coat as derived 

 from the dura mater. Many have also described the choroid as derived from 

 the pia mater. It is better, however, to adopt the opinion of Albinus. Winsknv, 

 and others, that these coats are peculiar and expressly adapted to the eye, and 

 that the sheath of the optic nerve, is continuous with the sclerotic without 

 abruptness, by a sort of insensible change, always seen when one membrane 

 terminates in another. This theory of formation, besides, is not borne out by 

 comparative anatomy ; for in scaly fish, where the dura mater is membranous, 

 the sclerotica, is corneous or bony. p. 



