THE FOOT 673 



combined, and it is this, in combination with hydrogen, which causes 

 horn undergoing disease to have such an offensive odour, sulphu- 

 retted hydrogen^being formed. Keratin is a very insoluble substance, 

 but is dissolved' by strong and boiling acids and by alkalies. With 

 sulphuric acid it yields leucine, tyrosine, and volatile substances, 

 the latter conferring the peculiar odour on burnt horn, feather, 

 nail, etc. 



Provisions^for Elasticity and Toughness. — From what has 

 previously been said, it can be seen that it is the wall of the foot 

 which supports the horse's weight. On examining the wall, it is 

 found to be thickest at the toe, thinner at the quarters, and 

 thinnest at the heels (Fig. 239); it is thickest at the toe owing to 

 the functions performed by this part, leading to excessive wear 

 and tear. As the pad and posterior part of the foot are the first 

 to make contact with the ground (at any rate, in all fast paces), 

 so the toe is the last part to leave it, the final propulsion being 

 given by it to the body, as we have seen in studying locomotion. 

 The object of the wall becoming thin towards the posterior 

 part of the foot is to allow of the elastic movement which has 

 yet to be described. Two physical conditions have to be pro- 

 vided for in the wall — i.e., elasticity of the posterior part and 

 toughness of the anterior portion. The first is furnished by 

 the wall being thinner at the heels than elsewhere ; but besides 

 being thinner, the wall of the heel contains more moisture than 

 the wall of the toe, and this moisture ensures its elasticity. 

 The younger the horn — viz., the nearer to the coronet at which 

 it exists — the more moisture it contains ; the farther away from 

 the coronet, the less moisture it possesses, and the tougher and 

 more resisting the horn. 



The wall grows evenly from the coronet all the way round ; 

 if it grows J inch in the month at the toe, it grows the same 

 length at the quarters, and the same at the heels. The anterior 

 part of the wall is longer than the posterior, therefore the anterior 

 is tougher than the posterior, for the reason that the horn is 

 much older at the extremity of the toe than it is at the heel, 

 and being farther away from the coronet, it contains less moisture. 

 The wall at the heel is some months younger than that at the 

 toe ; it is thinner and contains more moisture, therefore it is 

 more elastic, but not so tough. The age of the wall is an impor- 

 tant factor in the wear of the foot. If it takes from nine to twelve 

 months for it to grow from the coronet to the toe, the piece 

 of wall at /, Fig. 237, is, say, twelve months old, whilst that at 

 a' is less than six months old. The horn of the quarter is older 

 than the horn of the heel, and the horn of the toe older than 

 that of the quarter. This provision admits in the unshod foot 

 of considerable friction occurring at the toe without producing 



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