388 



ELEMENTAR Y ANA TOMY. 



[LESS. 



- -L.G. 



The ingrowth becomes pinched off, its hinder part forming 

 the crystalline lens, its anterior part the aqueous humour. 



A third process intrudes inwards from one side, between 

 these two growths, and forms the vitreous humour. 



The connective tissue around the whole structure becomes 

 condensed, as the sclerotic. 



The external skin develops folds which increase and 

 become the eyelids, and these are lined by a membrane re- 

 flected over the eyeball and called the conjunctiva. 



At first the optic nerves are separated, not forming a 

 chiasma. 



At the outer angle of the orbit is the lachrymal (or tear- 

 secreting) gland, the tears passing down a canal which 

 traverses the lachrymal bone as before noticed into the 

 nares. 



In the possession of eyes man agrees with the immense 

 majority of his sub-kingdom ; nevertheless, these parts 

 may be quite rudimentary and 

 covered by the external hairy 

 skin, as in the Mole. They 

 may even, as in Myxine, merely 

 consist of a minute lens in 

 contact with the end of the 

 optic nerve, which is coated with 

 pigment. 



The eye may be relatively 

 much larger than in man, as e.g. 

 in Tarsius, and in many Birds 

 and Fishes. 



The sclerotic may be more or 

 less ossified, as in Birds, Lizards, 



and many Fishes, but it is never so in man's own class. 

 The orbital muscles have been already described. 

 Part of the lateral process, which forms the vitreous 

 humour, may persist in part in its more primitive condition 

 as a distinct vascular membrane, covered with pigment, 

 extending from near the entrance of the optic nerve to the 

 lens. Such is the case in Birds and many lower forms, where 

 it is called \he pecten, or sometimes the marsupium. 



Lachrymal glands may be entirely absent, as e.g. in 

 aquatic forms Fishes and Cetaceans. 



Eyelids may be absolutely^ wanting, as in most osseous 

 Fishes ; or they may be apparently so, as in Serpents, where 

 they really exist in a transparent condition and quite with- 



FIG. 341. FRONT VIEW OF THE 

 EYE, WITH THE EYELIDS. 



Lachrymal gland,/, G; andlachry- 

 mal duct, LD. 



