Penile erections can also be caused by friction of the skin of the penis alone. This is known as a reflexogenic erection, which commonly occurs among men who have had damage to the spinal cord and who are unable to get an erection unless physically stimulated. The scrotum is a unique structure with very thin, loose skin that is slightly hair bearing. It is also controlled by the endocrine system and functions as a receptacle for the testes. Keeping the testicles outside the body produces a cooler environment and thus provides the best area for spermatogenesis, or the creation of sperm, to take place.
Warmth tends to prevent sperm from developing properly.
Underlying the skin of the scrotum is the cremasteric muscle. This muscle is incorporated into the scrotum and by contracting, it elevates the testicles. This is done in response to cool and warm weather and noxious or painful stimuli. The testicles are roughly the size of a small egg. They are responsible for the development of sperm as well as the manufacture of the hormone testosterone.