From Encyclopedia of Sex and Sexuality
The main organs of the male reproductive system. They are egg-shaped and paired and reside in a sac outside of the male body called the scrotum. In this location, they are kept about 4 degrees Celsius cooler than the rest of the body’s organs because they function better at the lower temperature.
The testicles have two separate but related main functions. Most of the testicle is devoted to production of sperm, the microscopic cells with tails that carry the man’s genetic potential to a woman’s egg. The other important function of the testicle (testis) is to manufacture the male sex hormones, primarily testosterone. At the time of puberty, the testicles receive a hormonal signal from the pituitary gland of the brain that tells them to start making testosterone. These hormones trigger the growth of a man’s pubic hair, beard, muscles, and penis, along with voice change and sex drive. Together with the brain’s hormone signal, the testosterone made by hormone-producing cells in the man’s testicles stimulates the reproductive cells (spermatogonia) to start dividing and producing sperm. In normal men sperm is produced at the rate of 50,000 sperm per minute, every minute of every day, from puberty until old age. The sperm manufacturing process takes about seventy-seven days, after which the sperm leave the testicles through a series of tiny ducts at the top and enter the epididymis, a 15-foot-long coiled duct, where they acquire the ability to swim and fertilize and egg. The sperm are then propelled from the epididymis to the penis by the vas deferens, a muscle-clad tube about fifteen inches long.
The testicles are very sensitive to heat, drugs, alcohol, radiation, and environmental toxins. Exposure to any of these can impair the ability of testicles to manufacture healthy sperm and hormones. About 15 percent of men have varicose veins called a varicocele surrounding their testes. These overheat the testicles and, over time, decrease a man’ fertility. Varicoceles can be safely repaired by an operation called varicocelectomy to prevent or treat male infertility. If one or both testicles do not descend into the scrotal sac by infancy (a condition called cryptorchidism, or an undescended testicle) it must be brought down with hormone shorts or surgery. Men with this condition may have reduced fertility and have a higher risk of developing cancer in the affected testicle.