108 DOMESTIC ANIMALS, DAIRYING, ETC. 



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. 



The Hoof. "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 sponta- 

 neous shortening of the wall occurs. 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 conductor of heat, and the harder (dryer) the horn, the more 

 slowly does it transmit extremes of temperature. 



Physiological Movements of the Hoof. A hoof while support- 

 ing the body weight has a different form, and the structures in- 

 closed within the hoof have a different position than when not bear- 

 ing weight. Since the amount of weight borne by a foot is continu- 

 ally 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 coffmbone 

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

 time the short pastern sinks backward and downward between the 

 lateral cartilages and presses the perforans tendon upon the plantar 

 cushion. This cushion being compressed from above and being un- 

 able to expand 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 car- 

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

 assisted and increased by the simultaneous flattening and lateral 

 expansion of the resilient horny frog, which crowds the bars apart. 

 Of course, when the lateral cartilages are ossified not only is no ex- 

 pansion of the quarters possible, but frog pressure often leads to 

 painful compression of the plantar cushion and to increase of lame- 

 ness. Frog pressure is therefore contra-indicated in lameness due 



