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 d ...

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 mus ...

enabling retraction 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

carnivoran Carnivora is a monophyletic order of placental mammals consisting of the most recent common ancestor of all cat-like and dog-like animals, and all descendants of that ancestor. Members of this group are formally referred to as carnivorans, ...

s, 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 Mino ...

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

"Genus summary for ''Tandonia''"

AnimalBase AnimalBase is a project brought to life in 2004 and is maintained by the University of Göttingen, Germany. The goal of the AnimalBase project is to digitize early zoological literature, provide copyright-free open access to zoological works, and pr ...

, last modified 23 May 2010, accessed 26 August 2010. In stallions, the retractor penis muscle is relatively underdeveloped. The retractor muscle 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