Male Reproductive Anatomy
Primarily, the male reproductive anatomy consists of the penis and testicles. The testicles are located between the hind legs of the stallion. The testicles are egg shaped and weigh about 150 to 300g each. They are abou 6-12 cm long, 5cm wide and 4.7cm hgh. They are covered in tough fibrous tissue known as tunica albugenia.
Sperm are very sensitive and need to be kept at a lower temperature than the rest of the body, therefore the scrotal sac holds the testes outside the body. In the unborn foetus the testes are held in the body cavity but around one month before birth, the testes descend into the scrotal sacs. These are aproximately 4-7 degrees centigrade cooler than the rest of the body.In warm weather a muscle known as the cremaster relaxes further to allow more heat to evaporate to avoid killing the sperm, and when it cold the muscle contracts and the testes are drawn closer to the body to conserve heat.
Occaisonally, a testicle may be retained and even if the horse is gelded he will continue to produce testosterone and exhibit stallion-like behaviour. He is, however, unlikely to be fertile, because the sperm cannot survive in high temperatures. This is a rig
The epididymis is a long, U-shaped convoluted tube which connects the seminiferous tubules, which produce sperm, to the vas deferens. The epididymis stores sperm and allows them to mature. The vas deferens is a muscular tube which carries the sperm the urethra, where they are ejaculated through the erect penis.
The bulbo-urethral (Cowper’s) glands lie either side of the urethra and flush the urethra with a clear fluid prior to ejaculation. The vesicular glands (located either side of the bladder) and the prostate gland (near the begining of the urethra) produce secretions which create a favourible, protective and nutritious medium for the sperm to travel in. This final product is called semen, and is ejaculated into the mare by the penis, the organ of copulation. When the stallion is sexually stimulated the penis doubles in resting size and fills with blood. It also becomes erect and ready for action.
Female Reproductive Anatomy
The vulva i the external part of the mare’s anatomy. This is the visible ‘bulge’ located below the anus. Between the labia (lips) is the opening to the vagina. This is a tube, about 10-15cm in diameter, and on average, 18-23cm long.
The walls of the vagina have a lining of mucus, and are very muscular to allow it to stretch for parturition (foalbirth). The vagina also acts as a self-cleaning structure, secreting acidic mucus that kills bacteria, and keeps reproductive anatomy clean. At the top of the vagina is a tight, thick walled sphincter muscle called the cervix, or ”neck”. It leads into the uterus (womb). At dioestrus (sexually inactive state) it is white, tight and 6-8cm long, 4-5cm wide. During oestrus, the muscles relax, and secretions increase, to allow the stallion’s penis to ease into the vagina. At this point, the cervix is pink in colour, thanks to increased bloodflow to the area.
The uterus is a hollow, vaguely T-shaped organ situated in between the fallopian tubes and the cervix. It is attached to the mare’s lumbar region by two large ligaments. The uterus is divided into the main body, and the uterine horns. The main body is 18-20 cm long and 8-12 cm in diameter, and in this area the foetus develops, while the horns are about 25 cm long and gradually reduce in diameter to 1-2cm as they approach the fallopian tubes. These are two tubes continuous with the uterine horns, approaching the ovaries, and gradually reducing in size. Between the uterine horn and the fallopian tube, there is a junction called the utero-tubular junction. This is where fertilization occurs, just on fallopian side. Fertilized ova control their own passage (it has been suggested they do this by localized oestrogen secretions), down the uterine horns, to settle in the uterine lining. Unfertilized ova remain at the junction, and gradually degenerate (over a period of a few months).
The ovaries are roughly kidney shaped, and during anoestrus, measure 2-4cm in length, 2-3 in width, and are hard to the touch, to to the absence of developing follicles. During the breeding season, they are softer to the touch, thanks to the developing fluid filled follicles, and measure 6-8cm in length, 3-4 in width.