PHYSIOLOGICAL ANATOMY OF THE EYEBALL. 



781 



its surfaces, a star with from nine to sixteen radiations extending from the centre to 

 about half or two-thirds of the distance to the periphery. The stars seen upon the two 

 surfaces are not coincident, the rays of one being situ- 

 ated between the rays of the other. In the foetus, the 

 stars are more simple, presenting only three radiations 

 upon either surface. These stars are not fibrous, like 

 the rest of the lens, but are composed of a homogene- 

 ous substance, which extends, also, between the fibres. 



The greatest part of the substance of the lens is 

 composed of very delicate, soft, and pliable fibres, 

 which are transparent, but perfectly distinct. These 

 fibres are flattened, six-sided prisms, closely packed 

 together, so that their transverse section presents a 

 regularly-tesselated appearance. They are from ^^ 

 to -g^nr of an inch broad and from ^-^-^ to -^-^ of an 

 inch in thickness. Their flat surfaces are parallel 

 with the surface of the lens. The direction of the 

 fibres is from the centre and from the rays of the stel- 

 late 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, are provided with exceedingly distinct, oval nu- 

 clei, with one or two nucleoli. These become smaller 

 as we pass deeper into the substance of the lens, and 

 gradually they disappear. 



The regular arrangement of the fibres of the lens makes it possible to separate its sub- 

 stance into laininaa, 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 peculiar organic nitrogenized sub- 

 stance, very analogous to globuline, called crystalline, combined 

 with various inorganic salts. One of the peculiar constituents of 

 this body is cholesterine. In an examination of four fresh crystal- 

 line lenses of the ox, we found cholesterine, in the proportion of 

 0*907 of a part per 1,000. In some cases of cataract, cholesterine 

 exists in the lens in a crystalline form ; but, under normal condi- FlG> 

 tions, it is united with the other constituents. 



FlG - ^-- 



1, crystalline lens; 2. 2, vit- 

 reous humor ; 8, 8. zone 

 of Zinn ; 4, 4, posterior 

 portion of tho zone of 

 Zinn. thrown into folds; 

 o. fi. 6. anterior and mid- 

 dle portions of the zone 

 ofZinu. 



Suspensory Ligament of the Lens (Zone of Zinn). When we come 

 to the description of the vitreous humor, we shall see that it occu- 

 pies about the posterior two-thirds of the globe, and is enveloped in 

 a delicate capsule, called the hyaloid membrane. In the region ot 

 the ora serrata of the retina, this membrane divides 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 corru- 

 gated portion is called the zone of Zinn. The two layers thus surround the lens and i 

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



