Scrotum

# Overview
In some male mammals, the scrotum is a protuberance of skin and muscle containing the testicles. It is an extension of the abdomen, and is located between the penis and anus. In humans, and some other mammals, the base of the scrotum becomes covered with pubic hair at puberty. In common speech, the scrotum is often improperly referred to as the testicles, which actually refer to organs encased inside the scrotum. The scrotum is homologous to the labia majora in females. In slang, the scrotum is often referred to as the "nut sack."

# Function
The function of the scrotum appears to be to keep the testes at a temperature slightly lower than that of the rest of the body. For the human, a temperature around 34.4 degrees Celsius (94 degrees Fahrenheit) seems to be ideal; 36.7 degrees Celsius (98 degree Fahrenheit) may be damaging to sperm count. The temperature is controlled by moving the testicles closer to the abdomen when it is cold, and away when hot.  This is done by the contracting and relaxing of the cremaster muscle in the abdomen and the dartos fascia (muscular tissue under the skin) in the scrotum. However, this may not be the main function. The volume of sperm produced by the testes is small, (0.1-0.2ml). It has been suggested that if testes were situated within the abdominal cavity that they would be subjected to the regular changes in abdominal pressure that is exerted by the abdominal muscles. This squeezing and relaxing would result in the more rapid emptying of the testes and epididymes of sperm before the spermatazoa were matured sufficiently for fertilisation. Some mammals do keep their testes within the abdomen and there may be mechanisms to prevent this inadvertent emptying e.g. elephants, sea mammals.
In most biological males, the cremaster muscle itself cannot be controlled voluntarily.  Contraction of the abdominal muscles, and changes in intraabdominal pressure, often can lift and lower the testicles within the scrotum.  Contraction of the muscle fibers of the dartos tunic (or fascia) is completely involuntary and results in the appearance of increased wrinkling and thickening of the scrotal skin.  The testicles are not directly attached to the skin of the scrotum, so this dartos contraction results in their sliding toward the abdomen.
Although the ideal temperature for sperm growth varies between species, it usually appears, in warm-blooded species, to be a bit cooler than internal body temperature, necessitating the scrotum.  Since this leaves the testicles vulnerable in many species, there is some debate on the evolutionary advantage of such a system.  One theory is that the impregnation of females who are ill is less likely when sperm is highly sensitive to elevated body temperatures.
An alternative explanation is to protect the testes from jolts and compressions associated with an active lifestyle. Animals that have 'stately' movements - such as elephants, whales and marsupial moles - have internal testes and no scrotum.'s

# Health issues
A common problem of the scrotum is the development of masses.  Common scrotal masses include
- Sebaceous cyst, also called an epidermal cyst
- hydrocele
- hematocele
- spermatocele
- varicocele
Other conditions include:
- contact dermatitis: may cause redness, swelling, and itching of the entire scrotum. Can result from soaps, solvents, detergents, and natural irritants such as poison ivy.
- inguinal hernia
- yeast infection
- swelling resulting from conditions external to the scrotum, including:
heart failure
kidney or liver disease
- heart failure
- kidney or liver disease

# Additional images
- Structure of the male genitalia.
- Vertical section of bladder, penis, and urethra.
- The superficial branches of the internal pudendal artery.
- A human scrotum, containing the testicles
A human scrotum, containing the testicles