THE FOOT 



6 7 : 



papillae found on the secreting surface, and (2) from the spaces 

 between the papillae. The papillae form tubular horn, the spaces 

 between them form intertubular horn (Fig. 234, b and c), and this 

 is arranged in an oval or concentric manner around the canal (Figs. 

 235 and 236), the cells composing it being so placed that their edges 

 are towards the papillae. There is, however, a layer of cells which 

 actually forms the wall of the canal, and these are arranged with 

 their sides next it ; or, to put it another way, they stand on their 

 edges. In the deep layer of the wall the papillae produce a much 

 greater secretion, and here the circular or oval masses of cells invest- 

 ing the canal are more prominent ; and, further, unlike those in the 

 anterior and middle parts of the wall, they need no reagent to 

 demonstrate their cellular nature (Fig. 235, c). If a section of wall be 

 stained with picro-carmine, only the canal contents of the external 

 and middle wall stain with carmine ; all the remaining substance 

 takes up the picric acid. In the deep wall this is different ; here the 

 whole of the cellular material secreted by the papillae is stained 



Fig. 236. — Microscopical Structure of Horn : Longitudinal Section of the 

 Wall. Low Magnification (after Lungwitz.) 



Note the different size of canals ; those on the right are nearest the laminae ; those 

 on the left are towards the outside wall ; they are smaller and more numerous 

 than those deeper seated ; d is a portion of a horn lamina. 



red, showing that these cells are protoplasmic rather than horny, 

 and partly accounting for the fact that this deep horn is always 

 softer than the middle or external horn of the wall. 



If a vertical section of horn be made, the canals are now seen divided 

 in their length (Fig. 236). Though spoken of as canals or tubes, they 

 are really not empty, but throughout their entire length contain cells 

 which are protoplasmic in nature. These, owing to the manner in which 

 they reflect light, give to the part a beaded appearance. The cells 

 contained within the canal are secreted by the apex of the papilla ; 

 they do not fill up the entire lumen of the canal. The use of the 

 canal system in horn is for the purpose of irrigation ; the horn must 

 be supplied with moisture ; the bulk of this is obtained through 

 these imperfect canals, the soft protoplasmic canal-wall readily 

 admitting transudation. It is not intended to assert that any- 

 thing like a fluid is circulating along the tubes, but moisture certainly 

 does find its way down, and is readily imbibed by, the surrounding 

 cells. Besides this arrangement for maintaining the moisture in 

 horn, there is no doubt that in the intertubular horn moisture passes 

 from the secreting surface from cell to cell, and in this way is trans- 

 mitted throughout the length of the foot. It can also be absorbed 



