276 



MANUAL OF HISTOLOGY. 



calcification of the dentine is commencing, we may remark the surface o* 

 the latter covered with already hardened but still short prisms (fig. 264, d}. 

 Not' seldom we encounter appearances as if over these prisms there were 

 superimposed a special cuticle, the so-called membrana prceformativa 

 (fig. 264, e). Such a membrane does not in reality exist however, and 

 the whole is only a deceptive appearance produced by the youngest layer 

 of enamel which is undergoing development, and which may often be 

 raised off in the form of a membrane (after the decalcification of the whole) 

 from the fully formed tissue beneath. 



13. Lens Tissue. 

 159. 



The crystalline lens (1) consists of a capsule enclosing a tissue formed 

 of extremely fine transparent fibres or tubules. The latter have had their 

 origin from the cells of the corneous embryonic plate, and the whole 

 structure bears a decidedly epithelial character. 



The capsula lentis (fig. 265, a) is a perfectly transparent membrane, 

 apparently structureless, and only under very high magnifying power 

 finely streaked. It is much thicker anteriorly than posteriorly (about 

 0-0135-0-0068 mm.). The inner surface of the anterior half of the 

 capsule is lined with flattened epithelium of simple nucleated cells, already 

 mentioned 87. These measure from 0*0169 to 0-0226 mm. in diameter 

 (fig. 265, b, and 269, d). 



In the neighbourhood of the zonula Zinnii this epithelium passes at its 

 external border into a zone of young cells with multiple nuclei and but 

 little cell body ; here also the thickening of the capsule ceases. 



Nearer still to the circumference we encounter (springing from these 



formative cells) roundish nucleated ele- 

 ments, destined to be transformed into 

 the fibres of the lens (Becker). 



The fibres of the lens or "lens tubes" 

 (Linsenrohren) (fig. 266, a, b) are pale and 

 transparent, without any further structure 

 in their interior. In the most external 

 layers of the lens they are especially trans- 

 parent, and measure in breadth 0-0902- 

 0-0113 mm., while in the central portion 

 of the organ they are finer (0-0056 mm.), 

 but more distinctly bounded and clearer. 



The fibres at the periphery (d) possess 

 a viscid homogeneous contents, probably 

 enclosed in a very delicate envelope, and 

 deserve, therefore, rather the name of 

 tubes. 



Those of the interior (b), on the other 

 hand, have become more solid, and not 

 unfrequently present a serrated appearance 

 along their border, a condition of great 

 significance, for the adhesion of the several 

 tubes one with another, especially among fishes, where these edges are 

 regularly toothed. 



As may be seen even from side views, the fibres of the lens are not 



Fig, 265. Diagrammatic sketch of the 

 human lens, a, the capsule; e, the 

 fibres of the lens, with widened ends, 

 4 applitd to the anterior layer of 

 epithelium b, and abutting behind 

 against the capsule e ; /, the so-called 

 nucleus zone. 



