Penis

Japanese: 陰茎 - いんけい

In animals that have internal fertilization, the male copulatory organ that transfers sperm into the female's body. It is not common in invertebrates, but is found in a large number of species, particularly terrestrial and parasitic animals. In vertebrates, it is common in mammals. The form varies depending on the species, but basically it consists of a pair of corpora cavernosa, the corpus spongiosum surrounding the urethra, and the glans penis. In some species, the urethra protrudes as a thread-like projection beyond the main body of the glans. The form of the glans penis is also highly variable, with some having papilla-like projections and others having horn-like projections. The glans penis is rich in sensory receptors. The corpus cavernosum plays an important role in the erection of the penis, and when a large amount of blood flows into the irregular, spongy cavity, the cavernous cavity expands due to blood pressure, becoming engorged and causing an erection. This type of penis belongs to the vascular type, and humans are a typical example. There is also the fibroelastic type, which is represented by cattle and other animals. In this case, the penis is relatively long and hard even at rest. This is because the fibrous membrane that covers the corpus cavernosum is dense and thick. The role of the corpus cavernosum during erection is not as important as that of the vascular type, and the expansion of the corpus cavernosum along with the relaxation of the contractile muscles running along the base of the penis play an important role. Some birds and reptiles also have penises, and many of them are projections arising from the wall of the cloaca, often with a groove for transporting semen.

[Yasumi Arai]

Human penis

It is located in front and below the pubic bone, and is a slightly flattened cylinder. It consists of the root of the penis, which is attached to the pubic bone, the body of the penis in front of it, and the particularly swollen part at the tip (glans). The average length from the body of the penis to the glans in Japanese men is 8 cm. The inside of the penis consists of two corpora cavernosa on the back and one corpus spongiosum on the underside, both of which are wrapped in a tough white membrane. The corpus spongiosum contains the urethra in its center, and the tip swells significantly to form the inner surface of the glans, and the urethra opens at the tip of the glans. In other words, the penis is the male copulatory organ, and also serves as the urinary tract and sperm tract. The corpus cavernosum has a sponge-like structure made of fibers running vertically and horizontally, and when it fills with blood it is an erection, and without an erection sexual intercourse is impossible. This erection is controlled by the autonomic nervous system.

[Masafumi Shirai]

[Reference] | Genitalia

Male Genitalia Structure
The left side of the figure is a side view, the right is a front view ©Shogakukan ">

Male Genitalia Structure

Source: Shogakukan Encyclopedia Nipponica About Encyclopedia Nipponica Information | Legend

Japanese:

体内受精を行う動物で雌の体内へ精子を送り込む雄性交尾器官をいう。無脊椎(むせきつい)動物には普遍的ではないが、かなり多くの種類にみられ、ことに陸生動物、寄生動物にこれを有するものが多い。脊椎動物では哺乳(ほにゅう)類に一般的である。形態は種類によって異なるが、基本的には1対の陰茎海綿体と尿道を取り巻く尿道海綿体、それに続く亀頭(きとう)からなる。なかには尿道が亀頭の本体よりさらに糸状突起として突出している種もある。亀頭の形態も、乳頭状突起のあるもの、角(つの)状の突出のあるものなど変異に富む。亀頭部には感覚受容器が豊富に存在する。陰茎の勃起(ぼっき)の際に重要な役割を果たすのは海綿体で、海綿状の不規則な管腔(かんこう)に大量の血液が流れ込むことによって、海綿腔が血圧で拡張し、充血して勃起がおこる。このようなタイプの陰茎は血管型に属し、ヒトがその代表的な例である。ほかに線維弾性型のものがあり、ウシなどに代表されるタイプである。この場合、静時でも陰茎の長さは比較的長く、固い。これは海綿体を覆う線維膜が緻密(ちみつ)で厚いことによる。勃起に際しての海綿体の役割は血管型ほどではなく、海綿体の膨大とともに陰茎の底部を走る収縮筋の弛緩(しかん)が重要な役割を果たす。なお、鳥類、爬虫(はちゅう)類にも陰茎を有するものがあり、多くは総排出腔壁より生じる突起で、しばしば精液輸送のための溝がある。

［新井康允］

ヒトの陰茎

恥骨部に接して前下方にあり、やや扁平(へんぺい)な円柱形で、恥骨に付着する陰茎根部と、それより前方の陰茎体部、先端のとくに膨大する部分（陰茎亀頭部）からなる。日本人の陰茎体部より亀頭部までの長さは平均8センチメートルである。陰茎の内部は背方に2個の陰茎海綿体と下面に存在する1個の尿道海綿体とからなり、いずれも強靭(きょうじん)な白膜によって包まれている。尿道海綿体はその中心部に尿道を含み、先端部は著しく膨大して亀頭内面を形成し、尿道は亀頭尖端(せんたん)部に開口する。すなわち陰茎は男性交接器であると同時に、尿路と精路を兼ねている。また海綿体は、縦横に走る線維よりなるスポンジ様構造をもち、ここに血液が充満した状態が勃起であり、勃起がなければ性交は不可能である。この勃起は自律神経によりコントロールされている。

［白井將文］

[参照項目] | 性器

男性性器の構造

出典　小学館　日本大百科全書(ニッポニカ)日本大百科全書(ニッポニカ)について　情報 | 凡例

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