FUNCTION OF THE SENSITIVE AVALL. 



109 



cells and these again are thrust off by continued growth, a 

 cyUnder must be formed. It would 

 seem, however, that the cells produced 

 by the tip of the papilla are different 

 in character to those produced by the 

 sides, and that at a very short distance 

 from the papilla itself the central cells 

 of the liorn cylinder begin to contract 

 so that spaces are left, much in the 

 same way as in the stalks of certain 

 grasses. As growth proceeds, the cen- 

 tral cells contract more and more, until 

 the original cvlinder becomes a veritable 

 tube. 



By examining a cross section of horn 

 laminae, like that shown in fig. 68, with 

 ^-inch objective, it is seen, even after 

 treatment with water alone, that the 

 lamina consists of two distinct parts. 

 The condition is better brought out by 



the use of alkalis or colouring materials. 



On treating the section with dilute 

 potash solution the outer part of the 

 lamina clears up and becomes almost 

 invisible, while the centre remains un- 

 altered, save that its cells become rather 

 more distinct. Again, the outer layer 

 of cells readily absorbs a carmine stain 

 while the inner refuses it. The outer 

 portion consists of young, uncornified 

 cells just secreted by the sensitive la- 

 minte. The central, darker part is 

 made up of cells which have already 

 become cornified. The function of the well-developed mucous 

 sheath (stratum mucosum) of the wall appears to be to facilitate 

 the downward movement of the wall from coronet to bearing 

 marsin. After disease of the sensitive wall the downward 

 growth appears impeded and the crust is apt to show defor- 

 mation. 



The function of the sensitive wall has been the subject of 



Fig. 68. — Transverse section of 

 two horn lamina) still connected 

 with the middle sheath of the 

 wall, a, middle sheath ; b, trans- 

 verse section of horn tubules ; 

 c, central cornified portion of 

 horn lamina ; d, e, and/, young 

 horn surrounding the fully cor- 

 nified parts. 



