CORNEA AXD YITEEOUS HOIOUR. 



707 



Cornea. — There is at first no aqueous chamber in the eye, and even 

 after the solution of continuity which gives rise to this space lias oc- 

 curred, the cavity is not dilated with fluid, till near the time of ])irth. 

 Even then it is very shallow and the lens is placed very near to the 

 cornea. The formation of the cornea is due to a differentiation of the 

 tissue in the layer of mesoblast wliich is introduced from the neigh- 

 bouring wall of the head, between the primitive lens-follicle and the 

 corneous epiblast, the cavity of the aqueous humour arising by the 

 separation of the corneous part from a layer of the mesoblastic tissue 

 lying within it. The latter gives rise to the anterior part of the 

 vascular capsulo-pupillary membrane, while a still deeper lamina closely 

 embracing the lens, remaining non-vascular, is converted into the lens 

 capsule. Along with the latter is also formed the suspensory ligament 

 of the lens. 



Vitreous humour. — The enlargement of the space for the vitreous 

 humour progTessing, the cells of the mesoblast which form its foundation 

 become stellated and very sparse from the effusion of a large quantity 

 of fluid, and the hyaloid membrane surrounding this structure takes its 

 origin from the same mesoblastic elements. 



Choroid and other membranes. — The mesoblastic substance which 

 surrounds the ocular vesicle externally is the source of a number of 



Fiir. 568. 



Fig. 568. — Blood-vessels of the Cap- 



SULO-PUPILLARY MEMBRANE OF A 



New-born Kitten, magnified (from 

 Kolliker). 



The drawing is taken from a prepara- 

 tion injected by Tiersch, and shows in 

 the central part the convergence of the 

 net-work of vessels in tiie pupillary 

 membrane. 



important parts. Among these 

 may be mentioned first the 

 choroid membrane, the cellular 

 (membrana fusca), fibrous, and 

 vascular layers of which are de- 

 veloped out of the deeper divi- 

 sion of the mesoblastic sub- 

 stance, and to the same source 

 may be traced in a later stage of 



development the ciliary processes, ciliary muscle and iris ; while the 

 zonula ciliaris may be regarded as a part of the deeper mesoblastic tissue 

 connected with the formation of the hyaloid membrane and membrana 

 capsulo-pupillaris. The folds of the ciliary processes gradually in- 

 creasing, encroach upon the space outside the margin of the lens and 

 indent the zonula ciliaris and canal of Petit. 



The sclerotic coat is due to a process of differentiation occurring in 

 the outer layer of the enveloping mesoblastic tissue, which occurs 

 considerably later than those which bring the choroid and the cornea 

 into existence, but there is from the first continuity between the corneal 

 tissue and that of the sclerotic coat. 



The capsulo-pupillary membrane, already referred to, may be looked upon 

 as at first a complete fibro-vascular investment of the lens, which owes its origin 



