THE LINEA ALBA. 49 



junction of the sole and wall. It is analogous to 

 the ' quick ' of the human nail, and is exceedingly 

 sensitive. 



In the untouched hoof, this line, which is as 

 vulnerable as the heel of Achilles, is thoroughly pro- 

 tected by the thick and strong horny layers of the 

 sole ; but if that protection be removed, or even 

 weakened, the pressure of the horse's weight against 

 a stone or similar object will cause such intolerable 

 agony, that the animal will drop as if shot when he 

 treads upon it. 



Now we will see how all these structures, which 

 are apparently so different, can work together in 

 harmony. 



Suppose the animal to be walking. At each step 

 a considerable part of the weight of the horse is 

 thrown upon the hoof. The first portion to come 

 to the ground is the elastic frog, and as the frog 

 yields to pressure, the sharp edged wall also comes 

 upon the ground, so that the horse is partly sup- 

 ported by the frog and partly by the wall. 



The pressure of the frog is transmitted to the 

 sole, and thence to the wall, which slightly expands. 

 There has been great controversy about this expan- 

 sion. Some writers, judging apparently from the 

 dried hoof, have denied that any expansion at all 

 takes place. But it must be remembered that the 



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