172 ANTHROPOLOGY. 



membrane or capsule. It differs but little from water in specific gravity 

 and refractive power. Chemically it consists of about ninety-eight parts of 

 water and two of chloride of sodium, with a trace of albumen. The space 

 occupied by this humor is divided into two apartments by the iris, anterior 

 and posterior. 



8. The Crystalline Lens is a transparent, double convex lens, the 

 anterior convexity less than the posterior. The former forms part of a circle 

 whose diameter is seven or eight lines in length, the latter one of four or 

 five lines. Its convexity varies, however, with age. A perfectly trans- 

 parent elastic capsule invests the lens proper. On opening the anterior 

 part of the capsule of an eye long dead, a liquid escapes, known as liquor 

 Morgagnii. This accumulation of liquid in all probability does not exist in 

 the recent eye. The crystalline lens itself is of quite complicated structure. 

 When boiled it can be separated into concentric layers, of which the exterior 

 are soft, the internal increasing in hardness to the central nucleus. These 

 lamin» appear to consist of parallel fibres united by finely serrated or 

 toothed margins. During the separation of these laminae, a moderate 

 pressure will cause the w^hole to part into three triangular or wedge-shaped 

 segments. In the recent lens, the density of the layers increases from the 

 surface towards the centre, this involving a corresponding increase in 

 refracting power. Consequently, rays of light falling on the exterior of 

 the lens, will be refracted to the same focus as those passing through the 

 centre, meeting there a more highly refracting medium. Certain defects of 

 vision depend greatly on abnormalities of the crystalline lens. Thus a con- 

 vexity greater than usual produces short-sightedness by causing the rays of 

 light to converge to a focus in front of the retina, thus necessitating an 

 unusual approximation of an object to the eye. In long-sightedness, the 

 lens being less convex than usual, the rays tend to a focus behind the retina, 

 and to bring this point of convergence on the retina, the object has to be 

 held at a greater distance. 



9. The Vitreous Humor, or the Hyaloid Body, fills the posterior 

 three fourths of the globe, and is in contact with the whole of the inner 

 surface of the retina. Anteriorly and centrally the crystalline lens is 

 imbedded in it, and more externally the ciliary processes are attached to it. 

 In the adult it is perfectly transparent, and consists of a membranous cap- 

 sule and areolar tissue secreting and inclosing a fluid like water. The 

 hyaloid membrane or capsule not only incloses the humor, but sends inwards 

 numerous processes, which join each other, and form a fine areolar or 

 cellular tissue, in the interstices of which the fluid is confined. These cells 

 all communicate with, each other, as a single puncture will permit the 

 gradual discharge of all the fluid. The vitreous body is traversed by a 

 small artery from the centralis retinae, extending to the capsule of the lens. 

 The vascular layer of the retina also in all probability sends into it vessels 

 and nerves. 



From, the preceding description it must appear that the eye is not 

 only of complete structure and delicate organization, but is also a refined 

 optical instrument. The rays of light proceeding from any object strike 



