The dartos fascia is a fat-free layer of smooth muscular fiber outside the external spermatic fascia

External spermatic fascia

The external spermatic fascia is a thin membrane, prolonged downward around the surface of the spermatic cord and testis. It is separated from the dartos tunic by loose areolar tissue...

 but below the skin

Skin

-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...

. It is a continuation of Scarpa's fascia

Fascia of Scarpa

The deep layer is a layer of the anterior abdominal wall. It is found deep to the Camper Fascia and superficial to the External Oblique muscle.-Anatomy:...

, which is a membranous layer of the subcutaneous tissue in the abdominal wall. The wrinkled (rugose) appearance of the scrotum is due to this layer of fascia.

Gender differences

• In males it is termed tunica dartos and lies beneath the skin
  Skin
  -Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
  
   of the scrotum
  Scrotum
  In some male mammals the scrotum is a dual-chambered protuberance of skin and muscle containing the testicles and divided by a septum. It is an extension of the perineum, and is located between the penis and anus. In humans and some other mammals, the base of the scrotum becomes covered with curly...
  
  . In older males the dartos muscle loses its tone, and tends to cause the scrotum to be smoother and to hang down farther.
• In females, the same muscle fibers are less well developed and termed dartos muliebris, lying beneath the skin of the labia majora
  Vulva
  The vulva consists of the external genital organs of the female mammal. This article deals with the vulva of the human being, although the structures are similar for other mammals....
  
  .
• It receives innervation from postganglionic sympathetic nerve fibers arriving via the ilioinguinal nerve and the posterior scrotal nerve. (http://anatomy.med.umich.edu/gastrointestinal_system/inguinal_tables.html)

Function

The tunica dartos acts to regulate the temperature

Temperature

Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...

 of the testicle

Testicle

The testicle is the male gonad in animals. Like the ovaries to which they are homologous, testes are components of both the reproductive system and the endocrine system...

s, which promotes spermatogenesis

Spermatogenesis

Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two...

. It does this by expanding or contracting to wrinkle the scrotal skin.

• Contraction reduces the surface area
  Area
  Area is a quantity that expresses the extent of a two-dimensional surface or shape in the plane. Area can be understood as the amount of material with a given thickness that would be necessary to fashion a model of the shape, or the amount of paint necessary to cover the surface with a single coat...
  
   available for heat loss, thus reducing heat
  Heat
  In physics and thermodynamics, heat is energy transferred from one body, region, or thermodynamic system to another due to thermal contact or thermal radiation when the systems are at different temperatures. It is often described as one of the fundamental processes of energy transfer between...
  
   loss and 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 cremaster muscle

Cremaster muscle

-Contraction:Its function is to raise and lower the testes in order to regulate the temperature of the testes and promote spermatogenesis. Contraction may also occur during arousal which can prevent injury to the testicles during sex....

 to elevate the testis but should not be confused with the cremasteric reflex

Cremasteric reflex

The cremasteric reflex is a superficial reflex observed in human males.This reflex is elicited by lightly stroking the superior and medial part of the thigh...

.

Related terms

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.