In some animals, the male

penis A penis (; : penises or penes) is a sex organ through which male and hermaphrodite animals expel semen during copulation (zoology), copulation, and through which male placental mammals and marsupials also Urination, urinate. The term ''pen ...

possesses a

muscle Muscle is a soft tissue, one of the four basic types of animal tissue. There are three types of muscle tissue in vertebrates: skeletal muscle, cardiac muscle, and smooth muscle. Muscle tissue gives skeletal muscles the ability to muscle contra ...

enabling retraction into the prepuce.

In particular animals

The retractor penis muscle occurs in marsupials and

carnivoran Carnivora ( ) is an order of placental mammals specialized primarily in eating flesh, whose members are formally referred to as carnivorans. The order Carnivora is the sixth largest order of mammals, comprising at least 279 species. Carnivor ...

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, 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 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