The dartos is a layer of smooth muscular fiber. In males it is termed tunica dartos and lies beneath the skin of the scrotum. In females, the same muscle fibers are less well developed and termed dartos mulierbris, lying beneath the skin of the labia majora.

The tunica dartos acts to regulate the temperature of the testicles in order to promote spermatogenesis. It does this by expanding or contracting to wrinkle the scrotal skin. Contraction reduces the surface area available for heat loss, thus reducing heat loss and thus warming the testicles. Conversely, expansion increases the surface area, promoting heat loss and thus cooling the testicles. The dartos muscle works in conjunction with the cremasteric muscle to raise the testis but should not be confused with the cremasteric reflex.

In older males the dartos muscle loses its tone, and tends to cause the scrotum to be smoother and to hang down further.

Some dartos-related terms:

dartoic (dar·to·ic) (dahr-to'ik) of the nature of a dartos; having a slow, involuntary contractility like that of the dartos.

dartoid (dar·toid) (dahr'toid) resembling the dartos.