In some animals the male

penis A penis (plural ''penises'' or ''penes'' () is the primary sexual organ that male animals use to inseminate females (or hermaphrodites) during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do n ...

possesses a

muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate muscular system and typically are attached by tendons to bones of a skeleton. The muscle cells of skeletal muscles are much longer than in the other types of muscl ...

enabling

retraction Retraction or retract(ed) may refer to: Academia * Retraction in academic publishing, withdrawals of previously published academic journal articles Mathematics * Retraction (category theory) * Retract (group theory) * Retraction (topology) Huma ...

into the

prepuce Prepuce , or as an adjective, preputial , refers to two homologous structures of male and female genitals: *Clitoral hood, skin surrounding and protecting the head of the clitoris *Foreskin, skin surrounding and protecting the head of the penis in ...

In particular animals

The retractor penis muscle occurs in

marsupials Marsupials are any members of the mammalian infraclass Marsupialia. All extant marsupials are endemic to Australasia, Wallacea and the Americas. A distinctive characteristic common to most of these species is that the young are carried in a po ...

and carnivorans, but it is absent in humans. A stag's penis forms an S-shaped curve when it is not erect, and is retracted into its preputial sheath by the retractor penis muscle. In

Tandonia ''Tandonia'' is a genus of air-breathing, keeled, land slugs. These are shell-less terrestrial gastropod mollusks in the family Milacidae. Distribution The distribution of the genus ''Tandonia'' includes Europe to North Africa and Asia Minor. ...

, the retractor penis inserts at the boundary between the penis and epiphallus.

"Genus summary for ''Tandonia''"
AnimalBase, last modified 23 May 2010, accessed 26 August 2010. In stallions, the retractor penis muscle is relatively underdeveloped. The

retractor muscle Retractor may refer to: * Retractor (medical), a medical instrument * Retractor (memory), a person * Retractor (chess), type of a chess problem * Retractor muscle Skeletal muscles (commonly referred to as muscles) are organs of the vertebrate ...

contracts to retract the penis into the sheath and relaxes to allow the penis to extend from the sheath. In bulls, protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the

sigmoid flexure The sigmoid colon (or pelvic colon) is the part of the large intestine that is closest to the rectum and anus. It forms a loop that averages about in length. The loop is typically shaped like a Greek letter sigma (ς) or Latin letter S (thus ''s ...

References

{{reflist Penis Muscular system