The testicles, known medically as testes (singular testis), are the male generative glands in animals. Male mammals have two testicles, which are often contained within an extension of the abdomen called the scrotum.
In mammals the testes are located outside of the body as they are suspended by the spermatic cord and within the scrotum. This is due to the fact that spermatogenesis is more efficient at a temperature somewhat less than the core body temperature of 37 degrees Celsius (99 degrees Fahrenheit). The cremasteric muscle is part of the spermatic cord. When this muscle contracts the cord is shortened and the testicle is moved closer up toward the body, which provides slightly more warmth to maintain optimal testicular temperature. When the temperature needs to be lowered, the cremasteric muscle relaxes and the testicle is lowered away from the warm body and are able to cool. This phenomenon is known as the cremasteric reflex. It also occurs in response to stress (the testicles rise up toward the body in an evolutionary effort to protect them in a fight) and they also contract during orgasm.
It is normal for one testis to hang lower than the other (usually the left). This is primarily due to differences in the vascular anatomical structure on the right and left sides. It is thought that this is another evolutionary development which protects each testis from bouncing off the other.
