Just to the rear of the terminal cone, the retracted penis shows (depending on the stage of retraction) a curve, varying from an S-shaped loop to a 360 degree circle. Because during erection the tough skin permits only a little, if any, stretching, this curve enables the animal to protrude the organ. A flat "retractor penis" muscle runs from just above the terminal cone to the rectal wall. This muscle is found only in species in which the penis is fixed to the abdomimal wall, such as ungulates, carnivores, and the guinea pig. Although it's presence is not always connected with a "fibro-elastic" penis, it is usually better developed in species with such a penis.
The muscle is not necessarily essential in the retraction of the penis, as studies have shown that the curve may naturally recur. The probable function is to act as a "brake" during erection.
The penis consists of three layers, those being an outer layer of thick, tough skin, a layer of connective tissue, and a rather insignificant core of elastic fibers. The urogenital canal is located on the lower side of the penis, within the connective tissue. During erection the inner layer fills with blood, but as mentioned this produces no noticable swelling.
It is suggested that the fibroelastic type coitus should be very rapid, with little or no friction movements. In general, this is true. In cattle, deer, antelope, sheep, goats, giraffes and okapis coitus varies from 2 to 5 seconds. However, in camels and llamas it may last up to 10 minutes. The quickest coitus in animals with a vascular penis is found in horses (30 seconds to 3 minutes) and cats (1 min). Most other species of this type last from 3 minutes to more than an hour. Cetaceans, however, do execute a quick coitus, averaging from 10 to 30 seconds.
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