In crocodilians (including alligators and large crocodiles), the phallus generally ranges from 4 to 10 inches (10 to 25 cm) in length depending on the overall size of the animal, though it remains completely hidden inside the cloaca when at rest. Anatomically, a crocodilian penis is built entirely of dense fibrous collagen tissue rather than erectile vascular tissue, making it permanently stiff even inside the body; instead of relying on blood pressure for an erection like mammals, it is "everted" (pushed outward) in a fraction of a second via specialized hydraulic muscles that contract to shoot the rigid organ out of the cloacal vent, and pulled back in by a elastic ligament when mating is complete.