The Muscles of the Penis
There are three muscles that aid the function of the penis.
15.3.1 Retractor penis muscle
This is a narrow, elongated development of largely smooth muscle. It is connected to the internal and external sphincter muscles of the anus and also with the coccygeal muscles.
The retractor penis muscle lies ventral to the body of the penis and has a loose attachment halfway along its length. The name of this muscle suggests an important function, but it is not essential for retraction of the penis into its sheath.15.3.2 Ischiocavernosus muscle
This powerful paired muscle consists of striated fibres. It originates from the ischiatic arch and inserts on the tunica albuginea (see Section 15.5.1), a thick layer of fibrous and elastic fibres surrounding the corpus cavernosum penis, and not to be confused with the fibrous layer enclosing the seminiferous tubules of the testis. The function of this muscle is to prevent venous blood from leaving the penis during erection. In addition, its rhythmic contractions increase the pressure within the corpus cavernosum penis so that erection can be complete.
15.3.3 Bulbospongiosus muscle
The bulbospongiosus muscle is a thick midline, circular, striated muscle. It overlies the bulb of the penis, i.e. the caudal enlargement of the corpus spongiosum. This muscle obstructs drainage of venous blood from the corpus spongiosum and raises the pressure by rhythmic contractions. The function of this muscle is to raise the pressure within the urethra by rhythmic contractions. Such pressure pulses constrict the urethra and result in ejaculation of the semen.
15.4