CRYSTALLINE LENS. 687 



the centre and from the rays of the stellate figures to the periphery, where 

 they turn and pass to the star upon the opposite side. The outer layers of 

 fibres near the equator, or circumference of the lens, contain exceedingly 

 distinct, oval nuclei, with one or two nucleoli. These become smaller in 

 passing more deeply into the substance of the lens, and gradually they dis- 

 appear. 



The regular arrangement of the fibres of the lens makes it possible to 

 separate its substance into laminse, which have been compared by anatomists 

 to the layers of an onion ; but this separation is entirely artificial, and the 

 number of apparent layers depends upon the dexterity of the manipulator. 

 It is to be noted, however, that the external portions of the lens are soft, 

 even gelatinous, and that the central layers are much harder, forming a sort 

 of central kernel, or nucleus. 



The lens is composed of a nitrogenized substance, called crystalline, com- 

 bined with various inorganic salts. One of the constant constituents of this 

 body is cholesterine. In an examination of four fresh 

 crystalline lenses of the ox, cholesterine was found in the 

 proportion of O907 of a part per 1,000 (Flint). In some 

 cases of cataract cholesterine exists in the lens in a crys- 

 talline form ; but under normal conditions it is united 

 with the other constituents. 



Suspensory Ligament of the Lens (Zone of Zinn}. 

 The vitreous humor occupies about the posterior two- FIG. 252. Zone ofzinn 

 thirds of the globe, and is enveloped in a delicate capsule, ^ crystalline lens ; 2, 2, 

 called the hyaloid membrane. In the region of the ora zone of^m^M, pos- 

 serrata of the retina, this membrane divides into two ^nTo^zinn? thrown 

 layers. The posterior layer lines the depression in the terfor'and'middie'p^r- 

 vitreous humor into which the lens is received. The an- SpJJf of the zone o 



/jllili. 



terior layer passes forward toward the lens and divides into 

 two secondary layers, one of which passes forward, to become continuous with 

 the anterior portion of the capsule of the lens, while the other passes to the pos- 

 terior surface .of the lens, to become continuous with this portion of its capsule. 

 The anterior of these layers is corrugated or thrown into folds which correspond 

 with the ciliary processes, with which it is in contact. This corrugated portion 

 is called the zone of Zinn. The two layers thus surround the lens and are 

 properly called its suspensory ligament. As the two layers of the suspensory 

 ligament separate at a certain distance from the lens, one passing to the ante- 

 rior and the other to the posterior portion of the capsule, there remains a 

 triangular canal, about ^ of an inch (2'5 mm.) wide, surrounding the border 

 of the lens, called the canal of Petit. Under natural conditions the walls of 

 this canal are nearly in apposition, and it contains a very small quantity of 

 clear liquid. 



The membrane forming the suspensory ligament is composed of pale, lon- 

 gitudinal and transverse fibres of rather a peculiar appearance, which are 

 much less affected by acetic acid than the ordinary fibres of connective tissue. 



Aqueous Humor. The space bounded in front by the cornea, posteriorly, 



