236 ANIMAL PHYSIOLOGY. 



The optic nerve, the retina, and the choroid, take rise from a 

 vesicular outgrowth of the brain, comparable with the olfac- 

 tory bulb (p. 204), and called the primary optic vesicle. The 

 neck of this vesicle remains as the optic nerve, while the 

 distal half of the vesicle becomes invaginated from below 

 upwards and backwards against the other half, so as to form 

 with it a double cup, the secondary optic vesicle, with a cleft 

 in its lower part. The pia mater or vascular covering in 

 the half nearest to the optic nerve is developed into the 

 choroid, and in the invaginated half remains as the retinal 

 artery; while the nervous matter of the first-mentioned part 

 disappears, and that of the invaginated portion is the main 

 substance of the retina. "Viewed in this light, the bacillary 

 layer and the hexagonal pigment cells of the choroid are 

 epithelial developments lining the opposed surfaces of the 

 optic vesicle.* The vitreous humour, sclerotic, cornea, and 

 iris, are later developments from subcutaneous tissue. 



174. The eye may be likened to a camera obscura, such as 

 that which is used in photography. In front are the refrac- 

 tive media by which the inverted image is produced, while, 

 behind, the retina receives that image precisely as the ground 

 glass or the sensitive plate in the artist's camera receives, 

 when the focus is rightly adjusted, the picture of the object 

 to be photographed. 



It must not, however, be supposed that the eyes of all 

 animals have the complexity of the eyes of vertebrate animals, 

 nor that vision is a sense enjoyed in perfection by all animals 

 possessed of eyes. The simplest forms of eyes met with, or 

 of structures which may be taken for eyes, are little more 

 than spots of colour; but even if we keep all doubtful struc- 

 tures out of view, eyes must be divided into those which are 

 capable of receiving an image of the landscape and those 

 which are not. A scallop (Pecteri) is provided with numerous 

 eyes, disposed like a double row of jewels, but it cannot 

 distinguish objects. Neither can the starfish do so, although 

 it has a group of eyes at the tip of every arm. In each of 

 these eyes there is a dense transparent structure or lens in 



* For tins reason I object to considering the hexagonal pigment 

 cells as belonging to the retina, as is done by Max Schultze, who lias 

 done so much to elucidate retinal structures. 



