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.
The Development of the Young.—We must here first speak of the ovum or germ, which is secreted by the ovary of the mare. Every time she comes in season (which occurs every three weeks during the hot weather) this ovum passes down the tubes before the womb as before mentioned, where it remains a few days and then dies if she is not put to the horse; but if, during the time this ovum is in the womb she is put to the horse and one of the spermatozoa from the semen of the horse comes in contact with it (the ovum) and a union of these takes place, then the rest of the semen dies and passes away, and the neck of the womb contracts gradually until it is perfectly tight. These two little bodies begin to grow when united and form the foetus, or foal. The foetus may be for convenience divided into three parts, viz.: the foetus proper, the navel string, and the placenta. The placenta is the part which is found covering the foal and is attached to the little pea-like elevations on the inside of the womb. This covering is found to be full of small blood vessels which finally unite to form two larger vessels, known as the navel veins. These carry the blood up through the navel opening of the foal and then to its heart. By the action of the heart it is forced all through the body of the foal and returned again to the heart. It is then forced down another artery to the navel opening, along the navel cord, into the placenta again, where it is distributed through the small blood vessels. When the blood comes down this cord from the foal it is in its impure state, and while it is passing through these small vessels in the placenta it comes very close to the small blood vessels in the womb. The blood is cleansed and nourished from the blood of its mother by a process similar to that which was spoken of when describing the lungs. The foetus, or foal, does not grow so fast the first month as it does later on. At the age of seventeen weeks the first hair appears on the lips and the tip of the tail. Between the thirty-fifth and the fortieth week the foal begins to show signs of life, and is completely covered with hair. After this time it grows very rapidly and may be seen moving around by watching the flank closely. The mare carries her foal eleven months, but in some cases an aged mare has been known to carry her foal over twelve months. In rare cases young mares may lack a few days of eleven months.
The Average Periods of Gestation of Domestic Animals.