THE TESTES
The testes (the male gonads) are paired structures usually contained in the scrotum. The two testes are about equal in size, averaging 5x2x3 centimeters (2 x 0.8 x 1.2 inches) in adults, although one testicle generally hangs lower than the other. Most often, the left testis is lower than the right one, but in left-handed men the reverse is usually true. There is no significance attached to the relative height of the testes within the scrotal sac, but if one testis is considerably larger or smaller than the other there could be a medical problem and a doctor should be seen.
The testes are highly sensitive to pressure or touch. Some men find that light caressing or stroking of the scrotum or gentle squeezing of the testes during sexual activity is arousing, but many others are uncomfortable with touching in this region.
The testes have two separate functions: hormone and sperm production. The cells that manufacture hormones — most importantly, testosterone, which controls male sexual development and plays an important part in sexual interest and function — are called Leydig's cells. Sperm production occurs in the seminiferous tubules, tightly coiled tubes of microscopic size that collectively measure almost 500 meters (more than a quarter mile) in length. The entire process of sperm production takes seventy days. Unlike the female, who creates no new eggs after birth, the male produces sperm from puberty on, manufacturing billions of sperm annually.
A mature sperm is considerably smaller than the size of a human egg, being about 0.06 millimeter (1/500 inch) in length and thousands of times smaller than the egg in volume. Sperm are only visible with the aid of a microscope, which shows that they consist of three pieces: a head, a midpiece, and a tail. The head of the sperm contains genetic material (chromosomes) and a chemical reservoir (the acrosome). The midpiece contains an energy system that allows the sperm to swim by lashing its long tail back and forth.
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