826 



THE OEGANS OF SENSE. 



pars ciliaris and pars iridica retinae. The indentation of the optic cup extends as a groove 

 for some distance along the postero-inferior aspect of the optic stalk, forming what is termed 

 the chorioidal fissure (Fig. 701). Through this fissure mesoderm passes inwards between 

 the lens and the retina to form a part of the vitreous body, while the arteria centralis retinse 

 also becomes enclosed in it and so gains its future position in the centre of the optic 



Lens rudiment 



Optic cup 

 Optic stalk ' 



Cavity of 

 fore-brain 



Outer layer of optic cup 

 Inner layer of optic cup 



Lens 



Optic 



vesicle 



becoming 



cupped 



| Optic stalk 

 I Chorioidal fissure 

 Lens 



Arteria 

 centralis 



fissure 

 Lens 



FIG. 702. OPTIC CUP AND LENS VIEWED 



FIG. 701. SECTIONS THROUGH PORTIONS OF THE HEADS OF FCETAL RABBITS, to illustrate the connexion 

 of the optic cup with the fore-brain, and the invagination of the ectoderm to form the lens. 



nerve. The arteria centralis is prolonged forwards from the porus options through 

 the vitreous body, as a cone of branches, as far as the back of the lens. By the fifth 

 or sixth month all these branches have disappeared except one, the arteria hyaloidea, 

 which persists until the last month of foetal life, when it also atrophies, leaving only the 

 canalis hyaloideus to indicate its position. 



The vitreous body is developed between the optic cup and the lens, and is derived 

 partly from ectoderm and partly from mesoderm. It consists primarily of a series of 



fine protoplasmic fibres which project from the cells 

 of the retinal layer* of the cup and form a delicate 

 Optic stalk reticular tissue. At first these fibres are seen in relation 

 to the whole of the optic cup, but later they are limited 

 to the ciliary region, where by a process of condensation 

 .Chorioidal they appear to form the zonula ciliaris. When the 

 mesoderm reaches the cup through the chorioidal fissure 

 it unites with this reticular tissue to form the vitreous 

 body. 



The lens, at first in contact with the ectoderm 

 FROM BEHIND AND BELOW, to show from which it ig derived, is soon separated from it by 

 "rTo? th cenSsTe" mesoderm, and then consists of a rounded vesicle with 

 tinse (from model by Ziegler). epithelial walls. The anterior wall remains as a single 



layer of cells the anterior lens epithelium of the 



adult ; the cells of the posterior wall become elongated into lens fibres, and by the 

 forward growth of these the cavity of the vesicle is obliterated. This elongation into 

 lens fibres is greatest at the centre of the lens, while near 'the equator the fibres are 

 shorter, and here the gradual transition between the anterior epithelium and the lens 

 fibres is seen (Fig. 696). The lens becomes enveloped in a vascular tunic, which receives 

 its vessels from the arteria centralis retinse and from the vessels of the iris. The front 

 part of this tunic forms the membrana pupillaris, and this, like the rest of the tunic, 

 disappears before birth. 



The hollow optic stalk becomes solid by the thickening of its walls and, acquiring 

 nerve-fibres, is transformed into the optic nerve. These nerve-fibres are mostly centripetal, 

 and are derived from the nerve-cells of the retina ; but a few are centrifugal and have 

 their origin in the brain. The further development of the retina resembles, in certain 

 respects, that of the spinal medulla. 



Cameron states (Jeurn. Anat. and PhysioL, vol. xxxix.) that in the early stages of the 

 development of the inner or retinal layer of the optic cup all the structures, described by His as 

 being present in the spinal medulla of the human embryo, are to be found, viz., (a) spongioblasts, 

 (6) germinal cells, and (c) neuroblasts. 



