The scrotum (or scrotal sac) is a part of the external male genitalia located behind and underneath the penis. It is the small, muscular sac that contains and protects the testicles, blood vessels, and part of the spermatic cord.
The scrotum is divided internally into two compartments by a septum and each compartment contains a testicle. The scrotum protects the testicles from temperature changes. In order to insure normal sperm production, the scrotum keeps the testes at a temperature slightly cooler than the rest of the body by contracting or expanding. The cremaster muscle contracts when the scrotum is exposed to cold temperatures, pulling the scrotum closer to the body for warmth. When exposed to heat, the cremaster muscle relaxes, enabling the scrotum hang lower and further away from the body. This allows the scrotum to cool.
The testicles descend into the scrotum during the first year of life. If this does not occur, hormone injections of B-HCG or testosterone are given or surgery may be performed in order to bring the testicles into the scrotum. If left untreated, undescended testicles may lead to infertility in later life.