MOVEMENTS OF THE HOOP. 569 



rest on the branches of the frog. In unshod hoofs the bearing edge 

 of the wall, the sole, frog, and bars are all on a level; that is, the 

 under surface of the hoof is perfectly flat, and each of these structures 

 assists in bearing the body weight. 



With respect to solidity, the different parts of the hoof vary widely. 

 The middle layer of the wall is harder and more tenacious than the 

 sole, for the latter crumbles away or passes off in larger or smaller 

 flakes on its under surface, while no such spontaneous shortening of 

 the wall occurs. The white line and the frog are soft horn struc- 

 tures, and differ from hard horn in that their horn cells do not under 

 natural conditions become hard and hornlike. They are very elastic, 

 absorb moisture rapidly, and as readily dry out and become hard, 

 brittle, and easily fissured. Horn of good quality is fine grained and 

 tough, while bad horn is coarse grained and either mellow and fri- 

 able or hard and brittle. All horn is a poor conductor of heat, and 

 the harder (drier) the horn, the more slowly does it transmit extremes 

 of temperature. 



THE PHYSIOLOGICAL MOVEMENTS OF THE HOOF. 



A hoof while supporting the body weight has a different form, and 

 the structures inclosed within the hoof have a different position than 

 when not bearing weight. Since the amount of weight borne by a 

 foot is continually changing, and the relations of internal pressure are 

 continuously varying, a foot is, from a physiological viewpoint, never 

 at rest. The most marked changes of form of the hoof occur when 

 the foot bears the greatest weight, namely, at the time of the greatest 

 descent of the fetlock. Briefly, these changes of form are: (1) An 

 expansion or widening of the whole back half of the foot from the 

 coronet to the lower edge of the quarters. This expansion varies 

 between one-fiftieth and one-twelfth of an 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. 



The changes of form occur in the following order : When the foot 

 is set to the ground the body weight is transmitted through the bones 

 and sensitive and horny leaves to the wall. The coiBn bone and navic- 

 ular bone sink a little and rotate backward. At the same time the 

 short pastern sinks backward and downward between the lateral carti- 

 lages and presses the perforans tendon upon the plantar cushion. 

 This cushion being compressed from above and being unable to ex- 

 pand downward by reason of the resistance of the ground acting 

 against the horny frog, acts like any other elastic mass and expands 

 toward the sides, pushing before it the yielding lateral cartilages and 

 the wall of the quarters. This expansion of the heels is assisted and 

 increased by the simultaneous flattening and lateral expansion of the 



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