CHEMICAL CONSTITUTION OF HORN. 



99 



lamina, in a somewhat oblique direction from fixed border to 

 free edge, or downwards and inwards. The tendency of the 

 homy laminae always to tear in this direction appears due to 

 the arrangement of their cells (tig. 56, /). 



The exceedingly fine particles of brownish, blackish, or deep 

 black material which the microscope always shows to be 

 present in greater or less amount between the cells of the 

 tubes and inter-tubular horn 

 is pigment. As its presence 

 interferes greatly with micro- 

 scopical examination, it is best 

 to employ uncoloured hoofs. 



. This pigment appears to serve 

 no particular purpose in the 

 construction of the hoof, the 

 colour of which varies from 

 white, yellow, or grey to a deep 



black aCCOrdincr to the amount fig. 65. -Pei-pendiculai- section of horn frog. 



. , . a, horn tubes; 6, cells of the inter-tubular 



of piSfmentatlOn. As the pier- horn, which are seen to run at right angles 



^ ^. 5 to the horn tubes. 



ment is produced by the corium, 



striped hoofs are due to absence of pigment-forming cells in 

 certain recjions of the coronet. It is said that dark hoofs are 

 stronger than light, but this requires confirmation. 



The contents of the horn tubes consist of loosely packed, 

 incompletely cornified cells, with broken - down material. 

 Sometimes traces of blood are found in the tubes, especially 

 after severe bruises of the corresponding horn-secreting parts. 



A very few lines on the physical and chemical properties of 

 the horn must suffice. The horn forming the hoof, when fresh 

 or after soaking in water, is fairly elastic ; but after drying it 

 loses this property. It is a bad conductor of heat, and there- 

 fore protects the parts it covers from freezing in winter 

 weather and from burning during the fitting of a hot shoe. 

 Burning horn produces a thick smoke, which has a character- 

 istic smell resembling that of burning feathers. Acetic acid 

 acts least, nitric acid most on horn, the latter turning it soft 

 and yellow ; sulphuric acid produces its effect slowly and 

 renders the cells more distinct. Caustic alkalis (soda and 

 potash) dissolve the inter-cellular substance and break up the 

 horn into its component cells. Ammonia acts similarly : 



