The scrotum is a bag of skin. The position of the testes in the scrotum is controlled by the cremasteric muscles which contract to pull the testes up towards the body when cold and relax to lower the testes into the scrotum when hot. By adjustment in the height of the testes their temperature is kept slightly lower than that of the rest of the body. This is important because normal sperm development can occur only if the testes are maintained at a temperature of about 2-3°F lower than that of the core of the body. Both fear and cold cause the cremasteric muscles to contract and so draw the testes nearer to the body. A mass of veins surround the artery that supplies the testis on each side, producing a heat-loss system which also helps to reduce the temperature of the testes. In this way the testes receive blood at a lower temperature than that supplied to the rest of the body.

Each testis is composed of several hundred little lobes, each of which contains a number of highly convoluted semeniferous tubules. These eventually straighten out and converge to form the five or seven ducts that open into the epididymis. The testis produces not only sperms but also makes hormones, the latter in special cells called Leydig cells. Testosterone is responsible for a man’s sexual characteristics (his penis, his muscular body, beard growth, distribution of body hair, deep voice and aggression), affects his metabolism and his psychological behaviour, and also stimulates the formation of various chemicals in his reproductive tract, that ensure the production of sperms.

The formation of sperms takes about two weeks. When they are first produced they are not motile (do not move). After passing through the tubules that form the testis, the sperms are collected in the epididymis where they are stored. While they are here they mature further and become motile. From the epididymis the sperms enter the vas deferens, a fine, muscular tube, and travel along this up out of the scrotum and into the abdominal cavity. It is the vas deferens each side that is tied off in a man undergoing vasectomy. The vasa deferentia gently milk the sperms along by muscular action to their upper ends where they widen to form the ampullae. Most sperms are stored in the epididymis, as we have seen, but the ampullae act as secondary storage sites. Beyond the ampullae are two blind bags off the vasa deferentia called the seminal vesicles. They are important because they produce a fluid containing a sugar called fructose which is the fuel sperms need to enable them to live on their journey to fertilise an egg.

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