SENSE ORGANS. 



447 



moored by " ciliary processes " of the choroid, and its shape 

 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 

 very complex structure with several layers of cells, partly 

 supporting and partly nervous ; the layer next the 



FIG. 143. Developmental of the eye. (After BALFOUR 

 and HERTWIG.) 



i. Section through first embryonic vesicle, showing outgrowth of 

 optic vesicles (pp.v.) to meet the skin ; f.b., fore brain ; G., the gut. 



2-4. The formation of the lens (/) from the skin, and the modifica- 

 tion of the optic vesicle into an optic cup ; ^., retina ; ?/./?., vitreous 

 humour. 



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



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 



