I530 A MANUAL OF ANATOMY 



radiate from its antero-posterior axis towards the circumference. 

 Scattered throughout the vitreous body there are some amoeboid 

 corpuscles, and it is traversed from behind forwards by a minute 

 passage called the hyaloid canal, or canal of Stilling (canal of 

 Cloquet). This canal extends from the centre of the optic disc to 

 the posterior wall of the capsule of the lens, and posteriorly it 

 communicates with the lymph-spaces of the optic nerve. In the 

 foetus the canal transmits a branch of the arteria centralis retinae, 

 called the hyaloid artery, which supplies the capsule of the lens. 



No vessels enter the vitreous body, its nutrition being derived 

 from the vessels of the retina and ciliary processes. 



Zonula of Zinn. — ^The zonula of Zinn, or zonula ciliaris, is the 

 thickened portion of the hyaloid membrane which is situated in 

 front of the ora serrata of the retina. From this point it extends 

 in^yards behind the ciliary processes towards the periphery of the 

 crystalline lens. Behind the ciliary processes it presents radial 

 folds with intervening depressions. The depressions receive the 

 ciliary processes, and the radial folds are separated from the 

 intervals between the ciliary processes by lymph-spaces, which 

 communicate with the posterior chamber of the eye. 



Suspensory Ligament of the Lens, and Canal of Petit. — ^The zonula 

 of Zinn, as it approaches the periphery of the lens, divides into three 

 layers, posterior, middle, and anterior. The posterior layer lines 

 the fossa patellaris on the anterior aspect of the hyaloid membrane. 

 The middle layer consists of a few scattered fibres which pass to 

 the equator of the lens. The anterior layer is the thickest, and 

 constitutes the suspensory ligament of the lens, which is attached 

 to the anterior waJl of the capsule of the lens not far from the 

 equator. When the radiating fibres of the ciliary muscle contract 

 the suspensory ligament is relaxed, and the convexity of the anterior 

 surface of the lens is increased. 



Behind the suspensory ligament of the lens there is a sacculated 

 lymph-space, called the canal of Petit, which surrounds the equator 

 of the lens. 



Development of the Eye. 



The retina, optic nerve, and crystalline lens are developed from the ecto- 

 derm, the retina and optic nerve being derived from the ectoderm of the 

 anterior primary cerebral vesicle, whilst the crystalline lens is developed 

 from the ectoderm of the side of the head. The accessories of the eye — e.g., 

 the sclerotic, cornea, choroid, ciliary body, and iris, are all developed from 

 the mesoderm. The vitreous body, though developed to a certain extent 

 from the mesoderm, is principally formed from the ectoderm. 



The earliest indication of the development of the eye is the formation of 

 a hollow protrusion on either side, from the lower and lateral portion of that 

 part of the anterior primary cerebral vesicle which forms the diencephalon or 

 thalamencephalon, anterior to the level of the infundibulum. This protrusion, 

 therefore, is an adjunct of the cavity of the diencephalon, which gives rise 

 to the third ventricle. On the ventral aspect of the diencephalon, between 

 the openings of the optic vesicles, there is a depression, called the optic recess, 

 in the posterior margin of which the optic ohiasma or commissure is developed. 

 The direction of each protrusion is outwards towards the ectoderm of the 

 side of the head. Towards its distal end it becomes enlarged, this enlarge- 



