HOKN TUBES AND INTER-TUBULAR SUBSTANCE. 105 



tween the little tubes thus produced. Eacli papilla, therefore, 

 furnishes the material for and produces a horn tube modelled 

 on its own form. The horn tubes correspond in size to the 

 papillaB producing them, the thicker papilhe producing larger 

 tubes. 



But this is only a part of the process, for were there no 

 cementing substance the innumerable tubes thus produced 

 would only loosely cover the sensitive foot, just as hair 

 covers a man's head or the mane a horse's neck, and therefore 

 we have to note that whilst the papilla- are producing tubes, 

 the surfaces between are secreting inter-tubular cells which 

 interlock with those of the tubes, and bind the whole into one 

 firm mass of horn. 



The strength and hardness of horn depend to some extent 

 on the age of the hoof. The older the cells the harder they 

 become ; hence the horn close to tlie secreting parts is softer 

 and more readily cut than that further removed. The degree 

 of toughness appears to depend largely on the arrangement 

 of the cells, being greater the more varying the direction 

 between the cells of the tubes and those of the inter-tubular 

 horn ; it is perhaps greatest in the softer horn structures 

 and middle sheatli of the wall, where the cells run in every 

 direction (compare fig. 65). The horn of the sole in which 

 the arrangement of the cells is entirely different exliibits little 

 toughness. 



Havint,^ obtained an idea of the method in w^hich the horn 

 of the wall, sole, and frog is produced and grows, it only 

 remains to note how the horny wall and horny sole are 

 <jonnected with their respective sensitive tissues. 



The wall is produced by the coronary band. The horn 

 grows downwards, surrounding and protecting the internal 

 portions of the foot. But it would be, connected with neither 

 the sensitive lamina nor with the horny sole were it not that 

 the lamina; have the property of producing horny material. 

 The lammal liorn is produced in comparatively small quantities, 

 and is not tubular. 



As there is no spot between the coronary band and sensitive 

 wall where the secretion of horn is interrupted, and as the 

 inter-papillary parts of the lower border of the coronary band 

 are continuous with the commencement of the sensitive laminae, 



