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 Organ (biology), 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 ...

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.

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 ...

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, 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 p ...

, 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