278 SENSATION AND THE SENSIFEROUS ORGANS. 



fications of the adjacent parts. Still more marvellous 

 is the history of the development of the organ of vision. 

 In the place of the eye, as in that of the nose and that 

 of the ear, the young embryo presents a depression of 

 the general integument; but, in man and the higher 

 animals, this does not give rise to the proper sensory 

 organ, but only to part of the accessory structures con- 

 cerned in vision. In fact, this depression, deepening 

 and becoming converted into a shut sac, produces only 

 the cornea, the aqueous humour, and the crystalline lens 

 of the perfect eye. 



The retina is added to this by the outgrowth of the 

 wall of a portion of the brain into a sort of bag, or sac, 

 with a narrow neck, the convex bottom of which is 

 turned outwards, or towards the crystalline lens. As 

 the development of the eye proceeds, the convex bot- 

 tom of the bag becomes pushed in, so that it gradually 

 obliterates the cavity of the sac, the previously convex 

 wall of which becomes deeply concave. The sac of 

 the brain is now like a double nightcap ready for the 

 head, but the place which the head would occupy is 

 taken by the vitreous humour, while the layer of night- 

 cap next it becomes the retina. The cells of this layer 

 which lie farthest from the vitreous humour, or, in other 

 words, bound the original cavity of the sac, are meta- 

 morphosed into the rods and cones. Suppose now that 

 the sac of the brain could be brought back to its origi- 

 nal form ; then the rods and cones would form part of 

 the lining of a side pouch of the brain. But one of 



