ORGANS OF SPECIAL SENSE. 353 



The structure of the lens consists of concentric laminae, made 

 up of hexagonal prisms, about one five-thousandth inch in 

 breadth, united laterally by dentated margins, and curving round 

 the borders of the lens. The laminas are arranged into three 

 triangular segments. 



The central portion, or nucleus, is unstratified, while the 

 outer layers, or cortex, each contain a nucleus, forming a nuclear 

 layer. 



The capsule of the lens is a clear, elastic, brittle membrane, 

 inclosing the lens, and held in position by the suspensory liga- 

 ment. Its anterior layer is thicker, and is attached to the lens by 

 a layer of polygonal nucleated cells, which break down post- 

 mortem to form the liquor Morgagni. 



The canal of Petit is about one-tenth of an inch in diam- 

 eter, encircling the capsule of lens. Its base is formed by the 

 capsule, and it is bounded in front by suspensory ligament, be- 

 hind by the hyaloid membrane of the vitreous humor. 



The suspensory ligament, or zonula of Zinn, is a thin, trans- 

 parent structure, extending from the margins of the hyaloid 

 fossa, where it is continuous with hyaloid membrane, to the an- 

 terior margin of the lens. 



THE VITREOUS HUMOR, or BODY, is a clear, albuminous fluid, 

 filling the cavity of the retina, hollowed out in front hyaloid 

 fossa for the lens, and inclosed in the hyaloid membrane. It 

 is composed of water, with a few salts, and a little albumen. 



Its structure is finely reticular, particularly in the foetus. 



The canal of Stilling,, canal of Cloquet, or hyaloid canal, 

 is a canal extending in the foetus from the entrance of the optic 

 nerve to the lens, containing fluid, and lined by a fold of hyaloid 

 membrane. 



In the foetus a canal extends parallel but independent of this, 

 and transmits a minute artery to the capsule of the lens. 



The hyaloid membrane is a delicate capsular membrane, 

 investing the vitreous body, excepting its anterior surface, sends 

 fibrous septa into the structure of the vitreous, and is continuous 

 in front with the posterior capsule, and with the anterior capsule 

 through the suspensory ligament. 



MUSCLES OF THE EYEBALL. 



UKCTUS SUPERIOR. Origin, sheath of the optic nerve and 

 upper margin of optic foramen; insertion, into upper surface of 

 sclerotic coat, three or four lines from corneal margin; action, 

 rotates the eyeball upward; nerve, third cranial. 



