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 7–8 cm in length and 3–4 cm thick, that generally tend to decrease in size as the mare ages. 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 cilia, which produce a current that flows toward the uterus. Each oviduct attaches to one of the two horns of the uterus, which are approximately 20–25 cm in length. These horns attach to the body of the uterus (18–20 cm long). Caudal to the uterus is the cervix, about 5–7 cm long, which enters the vagina. Usually 3.5–4 cm in diameter, it can expand to allow the passage of the foal. The vagina of the mare is 15–20 cm 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.
Stallion
The reproductive system of the stallion is responsible for the sexual behavior and secondary sex characteristics (such as a large crest) of the stallion. The external genitalia comprise:
- the testes, which are suspended horizontally within the scrotum. The testes of an average stallion are ovoids 8 to 12 cm long;
- the penis, within the prepuce, also known as the “sheath.” When not erect, the penis is housed within the prepuce, 50 cm long and 2.5 to 6 cm in diameter with the distal end 15 to 20 cm. When erect, the penis doubles in length and thickness and the glans increases by 3 to 4 times. The urethra opens within the urethral fossa, a small pouch at the distal end of the glans.
The internal genitalia comprise the accessory sex glands:
- vesicular glands;
- prostate gland; and
- bulbourethral glands.
These contribute fluid to the semen at ejaculation, but are not strictly necessary for fertility.
(Adapted from Wikipedia)