THE EYE. 761 



beginning of the third day, the formation of tlie Jens 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 secondare/ ocular vesicle 

 encompasses the lens, which in the mean time has been separated by 

 constriction from the epidermis, and comes into exact apposition with it 

 beneath, like a cup ; subsequently, however, the vitreous hod)/ is developed 

 between the two, in a special new cavity. How the latter is formed has 

 not yet been ascertained, although, as Schciler observes, it is most pro- 

 bable 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 E,emak, the 

 retina is formed from the inner, thicker wall of the inverted or second- 

 ary ocular vesicle, and from the outer and thinner, the clioroid, from 

 the anterior border of which the iris is not produced till afterwards. 

 The sclerotic and cornea are applied from without upon the eyeball 

 thus constituted, the former being to some extent a production of the 

 skin. 



An interesting phenomenon is presented in the vessels existing in the 

 foetal eye, even in the transparent media. The vitreous body, on its 

 outer surface, between the hyaloid membrane and the retina, presents a 

 tolerably wide-meshed vascular plexus, which is supplied by branches 

 of the arteria centralis retince, given oflF from it at its entrance into the 

 eye, and anteriorly, at the border of the lens on the zoyiula Zinnii, 

 forms a vascular circle, the circulus arteriosus Mascagnii, from which 

 again vessels are given off to the memhrana capsulo-pupillaris, pre- 

 sently to be described. Besides this, a special arteria liyaloidea, also 

 derived from the central artery of the retina, runs in the so-termed 

 canalis hyaloideus, in a straight line through the vitreous body, to the 

 lens, and ramifies in the most elegant arborescent manner, at very acute 

 angles, in a membrane closely applied to the posterior wall of the len- 

 ticular capsule. This is nothing else than a portion of an external vas- 

 cular capsule, which at first very closely surrounds the lens, and in its 

 anterior wall is supplied by the continuations of the hyaloid artery, 

 coming round the border of the lens towards the front, with which 

 branches of the circulus arteriosus Mascagnii and of the anterior 

 border of the uvea are connected. Afterwards, when the lens retreats 

 from the cornea, with which it is, at first, in close apposition, and the 

 iris buds out from the border of the uvea, the anterior wall of the vas- 

 cular lenticular capsule is divided into two portions : one central and 

 anterior, which, arising from the border of the iris, and connected with 

 that membrane by vessels, closes the pupil — the memhrana pupillaris ; 



