374 



THE BODY AT WORK 



which belongs to the pouch of brain that extended outwards 

 towards the eye-pit (p. 334). Properly speaking, therefore, it 

 is a layer of the retina. 



Three sets of tissues take part in the development of the 

 eyeball. (1) The epithelium covering the surface of the head 

 is depressed as a pit, which gradually closes into a hollow 

 sphere. This sphere, when its cavity is filled up, owing to the 

 great elongation of the cells of its posterior half, becomes the 

 lens. It breaks away from the rest of the epithelium of the 

 surface, which clears to transparency as that part of the con- 

 junctiva termed the " corneal epithelium." (2) The retina, as 

 already stated, is a hollow outgrowth from the interbrain. 



A B 



Fia. 28. DIAGRAMS SHOWING THE MODE OF FORMATION OF THE CRYSTALLINE LENS. 

 A, A pit in the epithelium on the surface of the head has closed into a hollow sphere. B, The 

 cells of the posterior wall of this sphere are growing forward, as the fibres of the lens 

 which traverse its whole thickness, with the exception of the cubical epithelium on its 

 front. 



As this pouch approaches the lens, its anterior half is pushed 

 back into the posterior half, forming a cup with a double wall. 

 The anterior, or inner, sheet of the bowl of the cup develops 

 into the nervous layers of the retina, the posterior sheet into 

 its pigmented epithelium. (3) Connective tissues are trans- 

 formed into the other constituents of the globe cornea, iris, 

 vitreous humour, etc. The globe is complete, except at a 

 spot on the nasal side of its posterior pole where the optic nerve 

 pierces it. 



The bloodvessels of the retina, entering with the optic nerve, 

 ramify on its anterior surface. Under ordinary circumstances 

 we ignore the shadows which they cast, as we ignore the blind- 

 spot which coincides with the disc of insensitive tissue presented 



