HOHSESHOEIXU. 587 



as a soft and very elastic wedge between the bars and between the 

 edges of the sole just in front of the bars. A broad and shallow de- 

 pression in its center divides it into two branches, which diverge as 

 they pass backward into the horny bulbs of the heel. In front of the 

 middle cleft the two brarfches unite to form the body of the frog, 

 which ends in the point of the frog. The bar of a bar shoe should 

 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 short- 

 ening of the wall occtirs. The white line and the frog are soft-horn 

 structures, 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 friable or hard and brittle. All horn is a poor con- 

 ductor of heat, and the harder (drier) the horn the more slowly 

 does it transmit extremes of temperature. 



THE PHYSIOLOGICAL MOVEMENTS OP THE HOOF. 



A hoof while supporting the J^ody 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 nar- 

 rowing 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 homy leaves to the wall. The coffin bone and 

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



