MANUAL FOR STABLE SERGEANTS. 49 



The lower or ground border comes in contact with the ground, and 

 is the part to which the shoe is fitted. Its inner surface is united 

 with the outer border of the sole by a soft, white horn, which appears 

 on the bottom of the foot as the so-called white line. 



(b) The sole is a thick, half-moon-shaped plate of horn forming 

 the greater part of the bottom of the foot. Its outer border is joined 

 to the inner part of the lower border of the wall by the previously 

 mentioned white line. 



Its inner border it V-shaped, and is attached to the bars, except 

 at its narrow part, where it joins the point of the frog. 



The use of the sole is to protect the sensitive parts above. It is 

 not intended to bear weight, except on a margin about one-eighth 

 of an inch wide inside of the white line. 



(c) The frog is a wedge-shaped mass of soft horn which occupies 

 the V-shaped space bounded by the bars and sole, and extends 

 below these on the bottom of the foot. 



On the lower or ground surface are two prominent ridges, separated 

 behind by a ca\dty called the cleft, and joined in front to form the 

 apex or point of the frog. 



The base or posterior extremity is depressed in the center and 

 bulged at the sides, where it unites with the wall at the heels, form- 

 ing two round prominences called the bulbs of the hoof. 



The upper surface of the frog is the exact reverse of the lower and 

 shows a middle ridge, the spine or frog-stay. Between the sides of 

 the frog and the bars are two cavities called the commissures. 



The frog protects the sensitive structures above, acts as a pad in 

 assisting the digital cushion in breaking jar, and prevents the foot 

 from slipping. The frog-stay forms a firm union between the frog 

 and the frog corium above. It may also assist in the expansion of 

 the foot by being forced like a wedge into the digital cushion when 

 the foot comes to the ground. 



EXPANSION AND CONTRACTION OF THE HOOF. 



116. Expansion. — ^When weight is placed on the foot it is re- 

 ceived by a yielding joint (coffin joint), an elastic wall, the rubber- 

 like frog, the digital cushion, and the more or less yielding sole. 

 The digital cushion and the frog are compressed between the ground 

 below and the structures above, which causes them to spread out 

 eidewise, carrying with them the cartilages -and bars and. the wall 



