THE BAR, THE EYE, THE SKIN, AND THE FOOT 183 



horseshoers, as it indicates the thickness of the wall. The inner 

 border of the sole is a -V-shaped notch which joins the bars except 

 at its narrow part where the frog articulates. 



The frog is an exact mold of the plantar cushion which it 

 protects. It is a wedge-shaped mass, filling up the angle between* 

 the bars and the sole, and extending considerably below these on 

 the ground surface of the foot. Thus the frog receives the great- 

 est amount of concussion, but it is seldom injured,' for itsJiorn is 

 of very elastic consistency. The ground surface presents a well- 

 marked median cleft, which corresponds to an elevation on the 

 superior surface known as the "frog stay" and which aids to 

 bind the frog to the overlying parts. 



The physiologic movements of the hoof are due to the fact 

 that the foot is never at rest. The constant shifting of the weight, 

 even when standing, changes the shape of the hoof. Adams has 

 grouped these changes of form into (1) An expansion or widening 

 of the whole back half of the foot from the coronet to the lower 

 edge of the quarters. This varies between y Xb and K2 inch. (2) 

 A narrowing of the front half of the foot, measured at the coronet. 

 (3) A sinking of the heels and a flattening of the wings of the sole. 

 These changes are more marked in the half of the foot that bears 

 the greater weight. In order to keep the foot healthful these 

 movements must be maintained with regularity. When the 

 horse is allowed to stand in the stable for days without exercise 

 the results are bound to show in the feet as pathologic changes. 



Variations in the activity of the growth of the hoof are shown 

 by more or less distinct ridges on the wall. These ridges run 

 parallel with the coronary border. The larger ones are thrown out 

 as a result of decided physiologic changes in the animal's 

 mechanism, such as foaling and a complete change in the charac- 

 ter of the feed or environment. The rings on the horns of cat- 

 tle are accounted for in the same way as the ridges on the wall 

 of horses' hoofs. 



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