PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 



781 



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 peculiar organic nitro- 

 genized substance, very analogous to globuline, called 

 crystalline, combined with various inorganic salts. 

 One of the peculiar constituents of this body is choles- 

 terine. In an examination of four fresh crystalline 

 lenses of the ox, we found cholesterine, in the pro- 

 portion of 0-907 of a part per 1,000. 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. 



FIG. 248. Section of the crystalline lens. 

 (Babuchin.) 



Suspensory Ligament of the Lens (Zone of Zinri). 

 When we come to the description of the vitreous hu- 

 mor, we shall see that it occupies about the posterior 

 two-thirds of the globe, and is enveloped in a delicate 

 capsule, called the hyaloid membrane. In the region 

 of the ora serrata of the retina, this membrane di- 

 vides into two layers. The posterior layer lines the 

 depression in the vitreous humor into which the lens 

 is received. The anterior 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 posterior 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 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. 



As we have already remarked in describing the retina, at the FlG " 249 ^ a py.f Zinn ' 



1, crystalline lens; 2, 2, vit- 

 reous humor ; 3, 3, zone 

 of Zinn ; 4, 4, posterior 

 portion of the zone of 

 Zinn, thrown into folds ; 

 5, 6, 6, anterior and mid- 

 dle portions of the zone 

 of Zinn. 



ora serrata, the membrane is closely connected, by a mutual inter- 

 lacement of fibres, with the suspensory ligament. It is important 

 to appreciate clearly the relations of the suspensory ligament, in 

 order to understand the mechanism of accommodation of the lens 

 to vision at different distances. The ciliary muscle being in repose, 

 during what is termed the indolent condition of the eye, when it is 

 adapted to vision at long distances, the tension of the parts flattens the lens ; but, in the 

 effort of accommodation for near objects, the ciliary muscle contracts, compresses the 

 contents of the globe, relaxes the suspensory ligament, and the inherent elasticity of the 

 lens renders it more convex. It is by a delicate use of this muscle, that the proper adap- 

 tation of the curvatures of the lens is obtained. 



The membrane forming the suspensory ligament is composed of pale longitudinal 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 by the crys- 



