THE EYE. 395 



§231. 



Physiological remarks. — The eye-ball is not developed from 

 a single point as a whole, but arises from the conjunction of 

 formations, proceeding on one side from the central nervous 

 system, on another from the skin, and, thirdly, from the parts 

 lying between the two. In the Chick, the primitive ocular 

 vesicles arise before the commencement of the second day, 

 from the primitive cerebral vesicle or the anterior cerebrum, 

 in the form of two protrusions, at first sessile, but after- 

 wards having a hollow peduncle — the rudiment of the optic 

 nerve. At the beginning of the third day, the formation of 

 the lens commences, from the skin of the face covering these 

 vesicles, by the thickening on the inner aspect, and inversion 

 of the epidermis, in consequence of which the anterior wall of 

 the primitive ocular vesicle is also inverted, and becomes 

 applied to the posterior wall, so that the cavity of the vesicle is 

 wholly obliterated. Now, at first, this secondary ocular vesicle 

 encompasses the lens, which in the meantime has been sepa- 

 rated by constriction from the epidermis, and comes into 

 exact apposition with it beneath, like a cup; subsequently, 

 however, the vitreous body is developed between the two, in a 

 special new cavity. How the latter is formed has not yet 

 been ascertained, although, as Scholer observes, it is most 

 probable that it also grows in from the skin, — in fact, from 

 the region below and behind the lens, — and participates with 

 the latter in the inversion of the primitive ocular vesicle. 

 According to Remak, the retina is formed from the inner, 

 thicker wall of the inverted or secondary ocular vesicle, and 

 from the outer and thinner, the choroid, from the anterior border 

 of which the iris is not produced till afterwards. The sclerotic 

 and cornea are applied from without upon the eye-ball thus con- 

 stituted, the former being to some extent a production of the skin. 



An interesting phenomenon is presented in the vessels exist- 

 ing in the foetal eye, even in the transparent media. The 

 vitreous body, on its outer surface, between the hyaloid mem- 

 brane and the retina, presents a tolerably wide meshed vascular 

 plexus, which is supplied by branches of the arteria centralis 

 retinae, given off from it at its entrance into the eye, and 

 anteriorly, at the border of the lens on the zonula Zinnii, forms 



