The scrotum is a sac that hangs under the penis and holds the testes. It is divided internally into two halves by a membrane; each half containing a testis. It has an outer layer of thin, wrinkled skin over a layer of tissue which contains muscle. The testicle lies inside the scrotum and produces as many as 12 trillion sperm in a male's lifetime, about 400 million of which are ejaculated in one average intercourse. 

Each sperm takes about seventy-two days to mature and its maturity is overseen by a complex interaction of hormones. The scrotum has a built-in thermostat, which keeps the sperm at the correct temperature. It may be surprising that the testicles should lie in such a vulnerable place, outside the body, but it is too hot inside. The sperm production needs a temperature which is three to five degrees below body temperature. If it becomes too cool on the outside, the scrotum will contract to bring the testes closer the body for warmth.