A study on Penis free essay sample

Penis (plural penises or penes) is a general term for the organs with which male and hermaphrodite animals introduce sperm into receptive females during copulation. Such organs occur in many animals, both vertebrate and invertebrate, but males do not bear a penis in every animal species, and in those species in which the male does bear a so-called penis, the penes in the various species are not necessarily homologous. For example, the penis of a mammal is at most analogous to the penis of a male insect or barnacle. The term penis applies to many reproductive intromittent organs, but not to all; for example the intromittent organ of most cephalopoda is the hectocotylus, a specialised arm, and male spiders use their pedipalps. In most species of animals in which there is an organ that might reasonably be described as a penis, it has no major function other than intromission, or at least conveying the sperm to the female, but in the placental mammals the penis bears the distal part of the urethra, which discharges both urine during urination and semen during copulation as the occasion requires. We will write a custom essay sample on A study on Penis or any similar topic specifically for you Do Not WasteYour Time HIRE WRITER Only 13.90 / page The Blue Whale has the largest penis of any organism on the planet, typically measuring 8-10 feet. Contents [hide] 1 In different animals 1. 1 Vertebrates 1. 1. 1 Mammals 1. 1. 2 Other vertebrates 1. 2 Invertebrates 2 Etymology 3 See also 4 References 5 External links In different animals Vertebrates Mammals Further information: Sexual reproduction in male mammals For the male human sexual organ, see Human penis. Penis of an Asian elephant. As with any other bodily attribute, the length and girth of the penis can be highly variable between individuals of the same species. In many animals, especially mammals, the size of a flaccid penis is smaller than its erect size. A bone called the baculum or os penis is present in most mammals but absent in humans and horses. In mammals the penis is divided into three parts:[1] Roots (crura): these begin at the caudal border of the pelvic ischial arch. Body: the part of the penis extending from the roots. Glans: the free end of the penis. The internal structures of the penis consist mainly of cavernous, erectile tissue, which is a collection of blood sinusoids separated by sheets of connective tissue (trabeculae). Some mammals have a lot of erectile tissue relative to connective tissue, for example horses. Because of this a horses penis can enlarge more than a bulls penis. The urethra is on the ventral side of the body of the penis. Stallions have a vascular penis. When non-erect, it is quite flaccid and contained within the prepuce (foreskin, or sheath). The retractor penis muscle is relatively underdeveloped. Erection and protrusion take place gradually, by the increasing tumescence of the erectile vascular tissue in the corpus cavernosum penis. Bulls, rams and boars have an S-shaped penis with a sigmoid flexure which straightens out during erection. Bulls have a fibro-elastic penis. Given the small amount of erectile tissue, there is little enlargement after erection. The penis is quite rigid when non-erect, and becomes even more rigid during erection. Protrusion is not affected much by erection, but more by relaxation of the retractor penis muscle and straightening of the sigmoid flexure. [2] Canids, including dogs, have a bulbus glandis at the base of their penis. During coitus the bulbus glandis swells up and results in a tie (the male and female dogs being tied together). Muscles in the vagina of the female assist the retention by contracting. Cats have barbed penises, with about 120–150 one millimeter long backwards-pointing spines. [3] Upon withdrawal of the penis, the spines rake the walls of the females vagina, which is a trigger for ovulation. As a general rule, a mammals penis is proportional to its body size, but this varies greatly between species – even between closely related ones. For example, an adult gorillas erect penis is about 4 cm (1. 5 in) in length; an adult chimpanzee, significantly smaller (in body size) than a gorilla, has a penis size about double that of the gorilla. In comparison, the human penis is larger than that of any other primate, both in proportion to body size and in absolute terms. [4] In the realm of absolute size, the smallest vertebrate penis belongs to the common shrew (5 mm or 0. 2 inches). Accurate measurements of the blue whale are difficult to take because the whales erect length can only be observed during mating. [5] Most marsupials, except for the two largest species of kangaroos, have a bifurcated penis, separated into two columns, so that the penis has two ends corresponding to the females two vaginas. [6] Neither marsupials nor monotremes possess a baculum. Echidnas have a four-headed penis, but only two of the heads are used during mating. The other two heads shut down and do not grow in size. The heads used are swapped each time the mammal has sex. [7] It has been postulated that the shape of the human penis may have been selected by sperm competition. The shape could have favored displacement of seminal fluids implanted within the female reproductive tract by rival males: the thrusting action which occurs during sexual intercourse can mechanically remove seminal fluid out of the cervix area from a previous mating. Other vertebrates Most male birds (e. g. , roosters and turkeys) have a cloaca (also present on the female), but not a penis. Among bird species with a penis are paleognathes (tinamous and ratites), Anatidae (ducks, geese and swans), and a very few other species (such as flamingoes). A bird penis is different in structure from mammal penises, being an erectile expansion of the cloacal wall and being erected by lymph, not blood. It is usually partially feathered and in some species features spines and brush-like filaments, and in flaccid state curls up inside the cloaca. The Argentine Blue-bill has the largest penis in relation to body size of all vertebrates; while usually about half the body size (20 cm), a specimen with a penis 42. 5 cm long is documented. Male turtles and crocodiles have a penis, while male specimens of the reptile order Squamata have two paired organs called hemipenes. Tuataras must use their cloacae for reproduction. [9] In some fishes, the gonopodium, andropodium, and claspers are intromittent organs (to introduce sperm into the female) developed from modified fins. The spine-covered penis of Callosobruchus analis, a Bean weevil. Invertebrates The record for the largest penis to body size ratio is held by the barnacle. The barnacles penis can grow to up to forty times its own body length. This enables them to reach the nearest female. [5] In male insects, the structure analogous to a penis is known as aedeagus. The male copulatory organ of various lower invertebrate animals is often called the cirrus. A number of invertebrate species have independently evolved the mating technique of traumatic insemination where the penis penetrates the females abdomen and deposits sperm in the wound it produces.