2.1.2 THE INTERNAL SEX ORGANS
The male Internal sex organs consist of the testicles which produce hormones and sperm, a system of ducts which transport and store the sperm, and some accessory organs whose secretions become part of the ejaculated semen.
The testicles (the male sex glands or gonads) are formed within the abdomen during the development of the embryo. However, before the birth of a baby boy they normally descend into his scrotum. In the mature male the testicles are two oval-shaped bodies of about 1 1/2 inches in length which are suspended in two separate sacs inside the scrotum outside the abdominal cavity. Although both testicles are of about the same size, the left one usually hangs a little lower and thus may give the appearance of being larger. The testicles serve a double function:
• They produce sperm which may be ejaculated through a system of genital ducts.
• They produce hormones which are secreted directly into the bloodstream.
The Production of Sperm
A testicle is composed of hundreds of small compartments which contain tightly coiled tubes not much thicker than a hair. Inside these tubes (called seminiferous tubules) the process of sperm production (called spermatogenesis) takes place. This process begins when the male reaches puberty and continues without interruption throughout his life. The production of sperm proceeds in three steps:
1. The first step begins with the cells that lie closest to the outer edge of the tube. These cells are called primitive spermatogonia and, like any other cell in the body, they have 46 chromosomes, including an X chromosome and a Y chromosome. By means of cell division a single primitive spermatogonium forms two new identical daughter cells. One of these takes the place of the original cell, while the other moves toward the center of the tube. This latter cell is called the primary spermatocyte.
2. The primary spermatocyte does not duplicate itself the way all other cells do, but divides in a unique way: It splits in half, as it were, allotting 22 chromosomes plus 1 X chromosome to one of the new cells, and 22 chromosomes plus 1 Y chromosome to the other. The two new cells are called secondary spermatocytes, and each of them contains only half as many chromosomes (23) as all other body cells.
3. The two secondary spermatocytes move even closer to the center of the tube, and each divides again in the ordinary fashion, duplicating itself exactly. The four new cells are called spermatids. These spermatids now change their shape, develop a tail, and thus grow into mature sperm cells called spermatozoa. The entire process through all three stages of development takes about 64 days. As is obvious from their origin and development, spermatozoa come in two varieties: those containing an X chromosome (and 22 other chromosomes), and those containing a Y chromosome (and 22 other chromosomes). In case of a fertilization the X-bearing spermatozoa will help to produce girls; the Y-bearing spermatozoa will help to produce boys. (For details, see "Conception.")
The Production of Hormones
As described in a previous section, the male and female gonads (testicles and ovaries) also produce certain hormones. These gonadal hormones have been divided into male hormones (androgens) and female hormones (estrogens). However, these terms are somewhat misleading because both "male" and "female" hormones can be found in every male and female body. It is only the quantity of these hormones that differs. Before puberty, the androgen and estrogen levels in boys and girls are nearly equal. Then, during adolescence, the balance begins to shift. In the male body the androgens rise to a slightly higher level than the estrogens, and in the female body the estrogens rise to a much higher level than the androgens.
In the male the increase of androgens during puberty helps to produce the secondary male sexual characteristics. In the female the increase of estrogens helps to produce the secondary female sexual characteristics. There is still much to be learned about the role of hormones in the human body. Nevertheless, a few basic facts have already been established:
While the gonadal hormones are necessary for a young person's physical maturation, they are not essential for the continued sexual activity of adults. In other words, males and females need the gonadal hormones during adolescence to develop their full sexual potential. However, once the potential has been attained they can function sexually without these hormones. This has long been recognized in the case of women whose gonads (the ovaries) cease functioning after menopause without diminishing their sexual responsiveness. Many people are less willing to concede that the same is also true for men who might be deprived of their gonadal hormones (by castration, for example). Indeed, in many countries adult males are still being castrated in the belief that this will eliminate their sexual capacities. However, this belief is erroneous. (For details, see "The Role of Hormones.")
THE SYSTEM OF GENITAL DUCTS
The sperm cells produced in the testicles are transported to their point of discharge from the body by a system of genital ducts. These ducts, which consist of matched pairs (in sequence: epididymides, vasa deferentia, ejaculatory ducts), lead from the testicles into the abdominal cavity where they eventually join the urethra, a single tube which discharges sperm as well as urine.
The sperm cells which are constantly being produced in the seminiferous tubules are moved into collection tubes which lie on the surface of each testicle. Such a collection tube is called epididymis (plural: epididymides}, and it is about 20 feet long. However, it is so twisted and convoluted that it seems no longer than the testicle itself. A sperm cell needs several weeks to traverse the collection tube. During this time it develops a limited ability to move by itself.
The Vasa Deferentia
Once the sperm cells have emerged from the collection tube, they enter a shorter and rather straight tube called vas deferens (plural: vasa deferentia). This tube leads from the scrotum into the abdomen. The lower portion of the vas deferens can be felt through the scrotal skin. Since it is so easily located, it can also easily be cut in a sterilization operation known as a vasectomy. (For details, see "Contraception.")
Inside the abdomen the two vasa deferentia (one associated with each testicle) bend in a long curve and lead up to a point behind the urinary bladder where they become enlarged, each forming a sort of sac or storage compartment called ampulla (plural: ampullae). The sperm cells are moved to these storage compartments to await ejaculation. The ampullae join the ducts of two other sac-like organs, the seminal vesicles, to form short and straight tubes called ejaculatory ducts. These ejaculatory ducts run inside the prostate gland and there join the urethra. (For details, see below.) Before entering the ejaculatory ducts, the sperm cells have only a limited capacity of moving by themselves. Instead, they are transported mainly by the movement of tiny hair-like structures inside the tubes and by muscular contractions. However, immediately upon ejaculation they begin to move very vigorously. This dramatic change is produced by several fluids from various sources which together make up the semen. Swimming in the semen, the sperm cells gain their full energy.
The urethra is a single tube which leads from the bladder to the tip of the penis. (The urethra should not be confused with the two ureters which lead from the kidneys to the bladder.) In the male, the urethra serves two important functions: to release either urine or semen. (Because of certain muscles, urine and semen cannot be released together.) While the urine enters the urethra directly from the bladder, the semen is composed of several different fluids which enter through special openings in the urethral wall mainly in the region of the prostate gland.
In order to survive after their ejaculation, the sperm cells need to swim in a thick, nourishing, protective fluid called semen. Actually, the semen is composed of several different fluids which come together at various points in the urethra. The most important of these fluids are produced by the organs described below.
The Seminal Vesicles
The seminal vesicles are two sacs which lie next to the ampullae (the enlarged endings of the vasa deferentia) behind the bladder and near the top of the prostate gland. It was formerly believed that the seminal vesicles just served as storage space for accumulated sperm. However, today the opinion prevails that their main function is to provide a fluid which, together with that of the prostate gland, activates the vigorous movement of the sperm cells after ejaculation.
The Prostate Gland
The prostate gland is a firm, round body about the size of a chestnut, and it lies directly below the bladder. It is traversed by the urethra as well as the two ejaculatory ducts described earlier. The prostate constantly produces secretions. Some of these are passed off with the urine. Others make up the greater portion of the semen.
In some older men, the prostate gland enlarges, causing pressure on the enclosed part of the urethra and thus making urination difficult. In these cases, the removal of the prostate by surgery may become necessary.
The Bulbourethral Glands (Cowper's Glands)
Below the prostate gland, there are two small glands about pea size which during sexual excitement secrete a clear, alkaline fluid into the urethra. Often a small drop of this fluid can be seen at the opening of the penis well in advance of an actual ejaculation. It is not entirely impossible for the drop to contain some stray sperm cells. (This could account for the rare cases of pregnancy without ejaculation of semen.)
The semen discharged in an ejaculation (usually somewhat less than a teaspoonful) is composed of sperm cells and secretions from the epididymides, the seminal vesicles, the prostate gland, and the bulbourethral (Cowper's} glands. None of these fluids contains any harmful substances. People who swallow semen, whether by accident or on purpose, have no reason to fear any ill effects. Semen is usually thick and greyish-white in color. However, at times it may also be thin and rather watery. The exact amount, consistency, and composition of semen depends on the frequency of ejaculations.