Vertehrata. 



335 



is a space in which, the aqueous humour is secreted ; outside the 

 retina, the choroid with the iris, and the sclerotic, develop from 

 the surrounding connective tissue. 



From this accovmt it will be clear that the sensory, that is, the essential, part 

 of the vertebrate eye, has a diUerent origin from the retina of invertebrate 

 animals. The origin of the vertebrate eye may be imagined thus : originally a 

 vesicular evagination of the brain sei-ved for the perception of hght, and later a 

 lens, etc., developed and closed the optic vesicle, which had been modified in the 

 meanwhile. Cf. also the pineal eye described below, which is similarly a vesicular 

 evagination of the brain. It must also be mentioned here that in certain lower 

 forms from which eyes are absent, part of the central nervous system is sensitive 

 to hght (see p. 22). 



The retina of the Vei'tebrata, hke that of many other animals, possesses 

 rod-hke refringent bodies which form a special layer (rods and cones) ; it differs 

 however, in that the rods are not in that part of the retina which is towards 

 the hght. They he directly upon the pigment sheath, and in order to reach 

 them the hght has to traverse all the other layers of the retina. A considerable 

 munber of thin layers is distinguished in the fully developed retina. 



The eye lies in a deep basin-like cavity on the side of the head 

 surrounded by loose connective tissue; this depression, the orbit, 

 is arched over above by the skull ; frequently too, it is more or less 

 completely bounded in front, below, and behind by osseous or carti- 



Mg. 276. Diagrammatic section througli the orbit A of a Fish, B oi a Mammal^ 

 c conjunctiva palpebrarum ; c' c. ocuU ; h cornea ; I upper ; I' lower eyehd ; m superior, 

 m' inferior rectus muscles ; n optic nerve ; o bulb of the eye ; 7* retractor bulhi. The skin 

 is represented by a thick black line, the connective tissue dotted, the wall of the orbit 

 (consisting of bones, muscles, etc.) shaded by oblique Unes). — Orig. 



laginous pieces. The cornea is generally directly continuous with 

 the skin, which is usually soft and flexible in this region, so that the 

 eye can move within the orbit without hindrance. The movements 

 are brought about by muscles, which arise from the skull, and are 

 attached to the eye-ball. There are almost always four rectus eye 



