3^4 



PRACTICAL HISTOLOGY. 



[XXXII. 



(5.) Two layers of pigment-cells the uvea the outlines of the 

 cells difficult to define, and continuous with the epithelium of the 

 pars ciliaris retinae. 



(6.) Blood- Vessels. A large number of blood-vessels and nerves 

 pass into the iris, but their distribution must be studied by the 

 methods already described for such purpose*. 



THE LENS. 



The lens is developed by the inflection of part of the epiblast, so 

 that it consists of modified epiblastic epithelial cells invested by a 

 transparent capsule. The capsule of the lens is a clear trans- 

 parent elastic membrane, not unlike a basement membrane, but is 

 less resistant to acids than the latter. Immediately on the inner 

 surface of its anterior part is a layer of clear flattened epithelial 

 cells hexagonal and nucleated the remains of the original cells. 

 Tracing these cells towards the margin of the lens they become 

 narrower and more cylindrical, until further on they gradually pass 

 into lens fibres. Posteriorly there are no epithelial cells between 

 the lens and its capsule, the lens fibres resting directly on the 

 capsule itself. The lens fibres are modified epithelial cells, the 

 fibre constituting the principal part of the cell. The cells are long, 

 soft, hexagonal prisms united to each other by more or less toothed 

 edges. The superficial fibres are longer and larger than the deeper 

 ones, and contain, or have on their surface, nuclei lying on the part 

 that corresponds to the equatorial (nuclear) zone of the fibre. No 

 nucleus is visible in the central fibres. 



9. Methods. Harden in its capsule, the lens of a cat or rabbit, 

 in Miiller's fluid (2-3 weeks) ; do not place it in spirit, but cut it 



in a freezing microtome, or cut in cel- 

 loidin. Paraffin is not suitable. Stain 

 the sections in eosin and mount in 

 Farrant's solution. Make meridional 

 and equatorial sections. 



T.S. Lens (a.) (LH.). Observe 

 the hyaline capsule if present and 

 just under its front part a layer of 

 epithelial cells, which, when traced 

 outwards, are seen to rapidly elongate 

 into lens-fibres (p. 364). Nuclei may 

 be seen on some of the fibres. 



(I.) T.S. of the fibres are hexagonal 

 (fig. 340, B). 



10. Isolated Lens Fibres Methods. (a.) Place the lens of a 

 frog or fish in a 10 per cent, solution of sulphocyanide of potas- 



FlG. 340. A. Fibres of the lens : 

 B. T.S. of the same. 



