The dartos muscle
is a muscle inside the scrotum
(the only one). It acts to regulate the temperature
of the testicles in order to promote spermatogenesis. It does so by expanding or contracting the scrotum. Contraction reduces the surface area
of the scrotum, thus reducing heat
loss and thus warming the testicles. Conversely, expansion increases the surface area, promoting heat loss and thus cooling the testicles.