THE EYE. 79 



are the canals of Stenson, and place these two cavities in 

 communication. Glandular diverticula of the mucous mem- 

 brane, supplied with nervous filaments from both the 

 olfactory and the fifth pair, may open into these canals. 

 They are called, after their discoverer, the " organs of 

 Jacobson." 



The Eye is formed by the coalescence of two sets of 

 structures, one furnished by involution of the integument, 

 the other by an outgrowth of the brain. 



The opening of the integumentary depression which is 

 primarily formed on each side of the head in the ocular 

 region becomes closed, and a shut sac is the result. The 

 outer wall of this sac becomes the transparent cornea^ of 

 the eye ; the epidermis of its floor thickens, and is meta- 

 morphosed into the crystalline lens ; the cavity fills with 

 the aqueous humour. A vascular and miiscular ingrowth 

 taking place round the circumference of the sac, and, 

 dividing its cavity into two segments, gives rise to the iris. 

 The integument around the cornea, growing out into a fold 

 above and below, results in the formation of the eyelids, and 

 the segregation of the integument which they inclose, as 

 the soft and vascular conjunctiva. The pouch of the con- 

 junctiva very generally communicates, by the lachrymal 

 duct, with the cavity of the nose. It may be raised, on its 

 inner side, into a broad fold, the nictitating membrane, moved 

 by a proper muscle or muscles. Special glands — the lachry- 

 mal externally, and the Harderian on the inner side of the 

 eyeball — may be developed in connection with, and pour 

 their secretion on to, the conjunctival mucous membrane. 



The posterior chamber of the eye has a totally distinct 

 origin. Very early, that part of the anterior cerebral vesicle 

 which eventually becomes the vesicle of the third ventricle, 

 throws out a diverticulum, broad at its outer, and narrow 

 at its inner end, which applies itself to the base of the in- 

 tegumentary sac. The posterior, or outer, wall of the diver- 

 ticulum then becomes, as it were, thrust in, and forced 

 towards the opposite waU, by an ingrowth of the adjacent 



