Penis of Male Animals: Structures, Muscles, and Types

The penis of male animals is a reproductive organ responsible for the transfer of sperm during copulation. It consists of various structures, including the shaft, glans, and in some species, an erectile tissue that allows for mating.

The penis is a copulatory organ in males, typically cylindrical in shape, extending from the ischiatic arch to the umbilical region, except in tom cats. It is supported by the penile fascia and skin.

In the prescrotal region, the penis is enclosed within the sheath or prepuce. The terminal portion, which moves freely within the sheath, is called the glans penis.

Parts of Penis

Penis of male animals has got three parts:

1. Root or base or phallus root
2. Body or phallus body
3. Tip of penis or glans penis of phallus tip

1. Phallus Root

The phallus root is also known as the root of the penis, penile root, or radix penis. It is the originating part of the penis, attached to the ischial bone and the ischiocavernosus muscle.

2. Phallus Body

The phallus body is also known as the body of the penis, penile body, or corpus penis. It is the middle part of the penis, formed by the fusion of two crura (corpora cavernosa) and the corpus spongiosum. It constitutes the major portion of the penis.

3. Phallus Tip

The phallus tip is also known as the tip of the penis, penile tip, or glans penis. It is the external part of the penis, attached to the body of the phallus. It forms a conical, tapering end with an almond shape.

Apart from the anatomical divisions into root, body, and glans, the penis can also be classified based on location into two parts: the pelvic (internal) part and the preputial (external) part. Additionally, it may be divided into protruded and non-protruded parts based on its protrusibility.

Types of Penis

The type of penis in male animals varies according to the ratio of connective tissue and erectile tissue. The penis of male animals can be grouped into two different types, depending on its internal microarchitecture.

1. Fibrous type
2. Vascular type

1. Fibrous Type (Musculofibrous / Fibroelastic Type)

The fibrous type of penis contains a high proportion of connective tissue. Well-developed fibrous tissue is a characteristic feature of bovines, porcines, ovines, and caprines.

2. Vascular Type (Musculocavernous Type)

Well-developed vascular sinuses and elastic fibers in the penis are characteristic features of equines, canines, and felines. The vascular type of penis contains a high proportion of erectile tissue.

The salient physiological functions of the penis are:

• To provide stiffness through the engorgement of numerous labyrinths of haemosinuses with the flow of blood upon proper sexual stimulation.
• To locate the vulva of the opposite sex during coitus.
• To perform intromission.
• To ejaculate semen into the female genitalia.
• To urinate.

Muscles of Penis

The body of the penis consists of the corpus cavernosum penis muscle, which is surrounded by a thick connective tissue capsule called the tunica albuginea.

Beneath the corpus cavernosum penis is the corpus cavernosum urethrae, which surrounds the urethral orifice. These two muscles are spongy in nature and are divided into many spaces. Erection of the penis is caused by the distention of these spaces with blood.

The root of the penis is formed by two crura that fasten the penis to either side of the ischiatic arch.

The ischio-cavernosus muscle, or erector penis muscle, is a short, paired muscle originating from the tuber ischii and sacro-sciatic ligament, and it is inserted on the crura and body of the penis. This muscle causes erection of the penis by its compressing and pumping action on the bulbous portion of the corpus cavernosum penis.

The retractor penis muscle is a paired smooth muscle that originates from the 1st and 2nd coccygeal vertebrae, divides, and meets again below the anus. It passes along the caudal ventral surface of the penis and attaches to the tunica albuginea of the penis. It draws the penis back into the sheath after ejaculation.

The muscles of the pelvic urethra include the urethral muscle, which is a circular muscle consisting of dorsal and ventral layers of transverse fibers. It aids in ejaculation and micturition by its forcible contraction.

The bulbo-cavernous muscle extends from the ischiatic arch to the glans penis. It is a continuation of the extra-pelvic urethra and is thickest at the root of the penis. It empties the extra-pelvic urethra.

Associated Structures

The penis of male animals has some specific associated structures.

Sigmoid Flexure

The sigmoid flexure is an ‘S’ shaped bend of the penis, which is characteristically found in cattle-bulls, buffalo-bulls, boars, rams, bucks, and some wild animals like giraffes.

Usually, it is post-scrotal in most species, except in boars, where it is pre-scrotal. Its posterior portion is attached to the retractor penis muscle.

After copulation, the sigmoid flexure returns to its original ‘S’ shape due to the contraction of the retractor penis muscle.

Os Penis (Os Priapi or Baculum)

The os-penis is characteristically found in male dogs, ferrets, and mink. It is occasionally present in tom-cats.

It is absent in other species. The size of the os-penis increases as the animal ages and may be used as an age indicator.

The os-penis of each species has a characteristic shape, which serves as a diagnostic taxonomic feature in certain livestock.

Corkscrew Penis

The corkscrew penis is a modified structure at the external extremity of the penis, which is a characteristic feature of the boar. This modification helps in the intromission of the boar’s penis into the genitalia of the sow.

Phallus

The phallus is the external part of the male genitalia, replacing the penis, which is a characteristic feature of the boar. This structure aids in ejaculation.

Phalli

The phalli are the external extremities of the male genitalia, replacing the penis, which is a characteristic feature of ducks and geese. This structure aids in intromission.

Penile Papillae

The penile papillae are located at the anterior portion of the penis, which is a characteristic feature of tom-cats, guinea-boars, hamsters, house mice, and rats. These papillae probably assist in ovulation.

Urethral Sinus

The urethral sinus is located around the end of the penis, which is a characteristic feature of the stallion. It contains the urethral sinus diverticulum.

Urethral Sinus Diverticulum

The urethral sinus diverticulum is located on the dorsal aspect of the urethral sinus, which is a characteristic feature of the stallion. It helps in the accumulation of debris and the formation of smegma, known as the “bean,” which is the main source of pheromones.

Urethral Process

The urethral process is a filiform appendage extending from the anterior portion of the glans penis, which is a characteristic feature in rams, bucks, and giraffes.

Blood Supply of Penis

• The internal pudendal artery supplies blood to the root of the penis.
• The obturator artery supplies blood to the body of the penis.
• The external pudendal artery gives rise to the dorsal artery of the penis after passing through the inguinal canal.

Nerve Supply of Penis

The autonomic nerves of the pelvic plexus, along with the hemorrhoidal and pudendal nerves, are responsible for innervating the penis. The sensory nerve fibers to the glans penis come from the dorsal nerve of the penis, a branch of the pudendal nerve. The glans penis is richly supplied with nerves and nerve endings.

Species Variance

Read about the species variance of the penis here (click here).