Accessory Sex Organs
The system of ducts that transports the spermatozoa begins with tubules inside the testis itself. From these tubules, the cells collect in a greatly coiled tube called the epididymis, which is 6 meters (20 feet) long and is located on the surface of the testis inside the scrotal sac (see Fig. 19-2). While they are temporarily stored in the epididymis, the sperm cells mature and become motile, able to move or “swim” by themselves. The epididymis finally extends upward as the ductus deferens, also called the vas deferens. This tube, contained in the spermatic cord, continues through the inguinal canal into the abdominal cavity. Here, it separates from the remainder of the spermatic cord and curves behind the urinary bladder. The ductus deferens then joins with the duct of the seminal vesicle on the same side to form the ejaculatory duct. The right and left ejaculatory ducts travel through the body of the prostate gland and then empty into the urethra.
Figure 19-2 Structure of the testis. The epididymis and spermatic cord are also shown.
Semen (meaning “seed”) is the mixture of sperm cells and various secretions that is expelled from the body. It is a sticky fluid with a milky appearance. The pH is in the alkaline range of 7.2 to 7.8. The secretions in semen serve several functions:
* Nourish the spermatozoa;
* Transport the spermatozoa;
* Neutralize the acidity of the male urethra and the female vaginal tract;
* Lubricate the reproductive tract during sexual intercourse;
* Prevent infection with antibacterial enzymes and antibodies.
The glands discussed next contribute secretions to the semen (see Fig. 19-1).
Figure 19-1 Male reproductive system. Organs of the urinary system are also shown.
The Seminal Vesicles: The seminal vesicles are twisted muscular tubes with many small outpouchings. They are about 7.5 cm (3 inches) long and are attached to the connective tissue at the posterior of the urinary bladder. The glandular lining produces a thick, yellow, alkaline secretion containing large quantities of simple sugar and other substances that provide nourishment for the sperm. The seminal fluid makes up a large part of the semen’s volume.
The Prostate Gland: The prostate gland lies immediately inferior to the urinary bladder, where it surrounds the first part of the urethra. Ducts from the prostate carry its secretions into the urethra. The thin, alkaline prostatic secretion helps neutralize the acidity of the vaginal tract and enhance the motility of the spermatozoa. The prostate gland is also supplied with muscular tissue, which, upon signals from the nervous system, contracts to aid in the expulsion of the semen from the body.
Bulbourethral Glands: The bulbourethral glands, also called Cowper glands, are a pair of pea-sized organs located in the pelvic floor just inferior to the prostate gland. They secrete mucus to lubricate the urethra and tip of the penis during sexual stimulation. The ducts of these glands extend about 2.5 cm (1 inch) from each side and empty into the urethra before it extends into the penis. Other very small glands secrete mucus into the urethra as it passes through the penis.
The Urethra and Penis
The male urethra, serves the dual purpose of conveying urine from the bladder and carrying the reproductive cells with their accompanying secretions to the outside. The ejection of semen into the receiving canal (vagina) of the female is made possible by the erection, or stiffening and enlargement, of the penis, through which the longest part of the urethra extends. The penis is made of spongy tissue containing many blood spaces that are relatively empty when the organ is flaccid but fill with blood and distend when the penis is erect. This tissue is subdivided into three segments, each called a corpus (body) (Fig. 19-5). A single, ventrally located corpus spongiosum contains the urethra. On either side is a larger corpus cavernosum (pl., corpora cavernosa). At the distal end of the penis, the corpus spongiosum enlarges to form the glans penis, which is covered with a loose fold of skin, the prepuce, commonly called the foreskin. It is the end of the foreskin that is removed in a circumcision, a surgery frequently performed on male babies for religious or cultural reasons. Experts disagree on the medical value of circumcision with regard to improved cleanliness and disease prevention. The penis and scrotum together make up the external genitalia of the male.
Ejaculation: Ejaculation is the forceful expulsion of semen through the urethra to the outside. The process is initiated by reflex centers in the spinal cord that stimulate smooth muscle contraction in the prostate. This is followed by contraction of skeletal muscle in the pelvic floor, which provides the force needed for expulsion. During ejaculation, the involuntary sphincter at the base of the bladder closes to prevent the release of urine. A male typically ejaculates 2 to 5 mL of semen containing 50 to 150 million sperm cells per mL. Out of the millions of spermatozoa in an ejaculation, only one, if any, can fertilize an ovum. The remainder of the cells live from only a few hours up to a maximum of 3 days.
Figure 19-5 Cross-section of the penis.