SENSE OF SIGHT. 



183 



isc to the lens. With the gradual obliteration of the artery, the 

 ipsule of the lens becomes thin and clears up. A foetus born ia 

 le seventh month is blind, because of the vascular and opaque 

 ipsule of the lens. The anterior part of the capsule— filling the 

 upil— is the membrana pupillaris. The part of the hyaloid 

 rtery within the optic nerve persists as the central artery of the 

 3tina. The canal of the artery within the vitreous humour, from 

 le optic disc to the lens, remains as the hyaloid canal — a lymph 

 ath. The hyaloid artery may persist and cause partial or com- 

 lete blindness. It disappears some days after birth in cats and 

 ibbits. 



(4) The Aqueous chamber, a space formed in the mesoblast which 

 es between the epiblast of the cornea and the lens (Fig. 148). Part 

 f this mesoblast becomes the anterior capsule of the lens ; part 

 ecomes the connective-tissue basis of the cornea. The aqueous 

 hamber is simply an enlarged lymph space formed between these 

 sro parts. Up to the time of birth the lens lies almost in contact 

 r ith the cornea (Fig. 149). 



hyaloid canal 



orbit, pi. front, 

 presphen. 



orbit, connect, tis. 



basis of Tenon's capr^f^^^ 



dental sacs. SU P- max. 



Fin. 140.— Section of the Eye and Orbit at birth. 



(5) The choroid, ciliary processes, and iris. — These form the 

 iddle or vascular tunic of the eye, and are developed out of the 

 esoblast which covers the optic cup. They form a vascular and 

 gmented covering through which the optic cup is nourished, 

 le ciliary muscle is formed in this tunic. 



