Reproductive System - Mare.

The reproductive system of the mare is responsible for controlling gestation, birth, lactation, as well as the estrous cycle and mating behavior of the mare. It lies ventral to the 4th or 5th lumbar vertebrae, although its position within the mare can vary depending on the movement of the intestines and distention of the bladder.
The mare has two ovaries, usually 7 to 8 centimeters (2.8 to 3.1 in) in length and 3 to 4 centimeters (1.2 to 1.6 in) thick, that generally tend to decrease in size as the mare ages. In equine ovaries, unlike in humans, the vascular tissue is cortical to follicular tissue, so ovulation can only occur at an ovulation fossa near the infundibulum. The ovaries connect to the fallopian tubes (oviducts), which serve to move the ovum from the ovary to the uterus. To do so, the oviducts are lined with a layer of cil which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately 20 to 25 centimeters (7.9 to 9.8 in) in length. These horns attach to the body of the uterus (18 to 20 centimeters (7.1 to 7.9 in) long). The equine uterus is bipartite,  meaning the two uterine horns fuse into a relatively large uterine body (resembling a shortened bicornuate uterus or a stretched simplex uterus). Caudal to the uterus is the cervix, about 5 to 7 centimeters (2.0 to 2.8 in) long, which separates the uterus from the vagina. Usually 3.5 to 4 centimeters (1.4 to 1.6 in) in diameter with longitudinal folds on the interior surface, it can expand to allow the passage of the foal. The vagina of the mare is 15 to 20 centimeters (5.9 to 7.9 in) long, and is quite elastic, allowing it to expand. The vulva is the external opening of the vagina, and consists of the clitoris and two labia. It lies ventral to the rectum. The mare has two mammary glands, which are smaller in virgin mares. They have two ducts each, which open externally.