450 



STRUCTURE OF VERTEBRATA. 



is alterable by the action of accommodating muscles 

 arranged in a circle at the junction of iris and sclerotic. In 

 many Reptiles, and in Birds, a vascular fold, called the 

 pecten, projects from the back of the eye into the vitreous 

 humour. A similar fold in Fishes {processus falciformis) 

 ends in a knot-like structure in the lens. The retina is a 



/.- 



Fig. 192. — Development of the eye. — After Balfour and 

 Hertwig. 



1. Section through first embryonic vesicle, showing outgrowth of 



optic vesicles (o^.v.) to meet the skin ; f.b. , thalamen-cephalon ; 



G., the gut. 

 -4. Sections illustrating the formation of the lens (/.) from the 



skin, and the modification of the optic vesicle into an optic 



cup ; R., retina ; v.k., vitreous humour. 

 5. External aspect of embryonic eye ; /., lens. 



very complex structure, with several layers of cells, partly 

 supporting and partly nervous ; the layer next the vitreous 

 humour consists of nerve-fibres, while that furthest from 

 the rays of light and next the pigment epithelium consists 

 of sensitive rods and cones. The region where the optic 

 nerve enters, and whence the fibres spread, is called the 

 blind spot, and near this there lies the most sensitive 

 region — the yellow spot, with its fovea centralis, where 



