SOLE OF THE FOOT.--The sole is a thick plate of horn which helps to form the under part of the hoof. It is situated between the inner border of the under part of the wall already mentioned and the front of the frog. The under part of the sole is concave, or hollowed out. The upper part of the sole is attached to the under surface of the os pedis bone, or bone of the foot, by a membrane called the quick, or sensitive sole--this membrane is merely a continuation of the sensitive laminae. The outer part of the sole is attached to the inner part of the wall. When pared down a white ring is seen where the sole and the wall unite. At the back part of the sole there is a notch the shape of the letter V; in this notch the frog is situated. It is important to remember when shoeing never to let the shoe rest on any part of the sole; neither is it well to pare off too much of the barky-looking substance of the sole, as this helps to keep the moisture in the foot. When this is taken off it allows the moisture to escape and the hoof becomes dry and contracted. FROG OF THE FOOT.--The frog is the important spongy horn found in the V-shaped notch in the back of the sole. It is wide at the back and helps to form the heel of the foot; the pointed part in front is called the apex of the frog. The under part of the frog is triangular in shape and has a hollow in it called the cleft of the frog. There is a hollow at each side of the frog, between it and the bars, called the commissures of the frog. On the upper part is a membrane, known as the sensitive frog, which attaches it to the under part of the os pedis, or foot bone. This membrane is simply a continuation of the sensitive sole spoken of in connection with the sole. The back part of the frog is the widest part and spreads out to form the heel. The study of the foot of the horse is of the greatest practical importance, owing to the many diseases and injuries to which it is liable. It resolves itself here into the consideration of the hoof or horny case, and the parts contained within it. CHAPTER VIII. EMBRYOLOGY.