354 NORMAL HISTOLOGY AND ORGANOGRAPHY. 



not been worked out very carefully, it is reasonable 

 to suppose that we have much the same structure as 

 in the human nail-bed, such as free nerve endings, 

 end-bulbs, and perhaps Rufini corpuscles. 



Hoof Horn. Like bone, this is tubular, re- 

 sembling Haversian systems, but, unlike bone, it 

 consists of compact layers of epithelial cells. As the 

 human nail develops from the germinal epithelium 

 at the root of the nail, so the wall of the hoof de- 

 velops from similar epithelium that covers the 

 coronary cushion situated at the upper margin of 

 the hoof wall. This cushion has an abundance of 

 epithelial papillae, and from the surface of these 

 papillae cells proliferate to form the wall of the hoof 

 tubes, while from the epithelium at the bases of 

 these papillae cells proliferate to form the hoof matrix. 

 Thus, the hoof horn is exclusively an epithelial tissue, 

 composed of flattened, scaly cells, with often an 

 easily detected nucleus, cemented together com- 

 pactly, their protoplasm being replaced by keratin 

 granules, a protein-like substance very insoluble and 

 containing 4.23 per cent, sulphur. The horn tubes 

 extend downward from the papillae of the coronary 

 cushion and are parallel to each other. These tubes 

 are smaller near the surface of the hoof and become 

 larger in the deeper portion. The scaly cells of the 

 tube wall are placed with their flat surfaces facing 

 the tubes, that is, their long axes are perpendicular, 

 while the long axis of the matrix cells are horizontal. 

 This conforms to their origin, the former prolifer- 

 ating from the sides of the vertical coronary papillae, 

 while the latter come from the horizontal coronary 



