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In the human reproductive system, germ cells have one basic function: to maintain life. Reproduction is the only way nature has of preventing life from becoming extinct.
The male reproductive system includes the testes, the vas deferens, certain glands (prostate, seminal vesicles and Cowper's glands), and the penis. The testes are two oval shaped bodies included in a sac, the scrotum, outside the body. Inside each of the testes are many tightly coiled tubes called the seminiferous tubules. The seminiferous tubules empty into ducts that lead to a structure called the epididymis, which is a convoluted tubule. The sperm pass from the epidymis into a long tube called the vas deferens. The vas deferens carries sperm from the testes to the urethra, the tube that leads out of the body. The two vas deferens open to the urethra through the prostate gland, which is a short distance from the urethra's opening into the bladder. The prostate gland is a small muscular gland about the size of a walnut. It produces a fluid that mixes with sperm before sperm leaves the body. The seminal vesicles are two glands located at the top of the prostate gland. They secrete a substance that helps the movement of the sperm.
Sperm are formed in the seminiferous tubules by a process of cell division of the epithelial cells lining the walls of the tubules. New sperm are formed almost continuously during the sexual cycle of the male, and the number is very great.
The male discharge, called semen, consists of sperm, of which there may be 300,000,000 in a single ejaculation. The fluid secretions of the other accessory glands form an alkaline medium, so that the sperm are not killed by the acid reaction of the urethra into which the semen is emptied. The semen is propelled through the seminiferous tubules and vasa deferentia by means of cilia, which line the lumen of the ducts, and by the muscular contraction of the walls of the vas deferens.
The urethra passes through the penis, which is the male copulatory organ. The penis consists of cavernous, or erectile, tissue, which is made up of connective tissue and numerous venous or blood spaces. The erection of the penis is made possible by the filling of the spaces with blood. The whole process is a nervous reflex action, the center of which lies in the lumbar part of the spinal cord. The entire process of male reproduction begins at puberty.
The female reproductive organs, or ovaries, begin functioning during puberty. The reproductive system includes the ovaries, uterus, fallopian tubes, vagina, and vulva. There are two ovaries lying within the abdominal cavity slightly below the level of the umbilicus. They are almond-shaped and are held in place and attached by a broad ligament extending laterally from the uterus. They are composed largely of connective tissue with a layer of germinal epithelium. The ova are formed from the germinal epithelium by division. The germinal epithelium dips down into the body of the ovary, and some of the epithelial cells become separated and surrounded with connective tissue. The ovum is made from one of the separated cells of the epithelial mass, and the others form a layer around this ovum. The ovum and its surrounding structure are known as a Graafian follicle. This follicle moves nearer the surface as the ovum nears maturity and ruptures to extrude the ovum to the surface of the ovary.
In the physically mature female there is usually one egg released from the ovary about every 28 days. The egg may be released from either ovary. The release of the egg from an ovary is called ovulation. After being released, the egg enters one of the fallopian tubes. These fallopian tubes originate from the angles of the uterus and extend out toward the ovaries. The open ends of the tubes are funnel-shaped with fringes. The tubes are lined with cilia, and their walls are muscular. Each of the two fallopian tubes has a cuplike structure at one end. This allows the fallopian tube to catch the egg as it is released from the ovary. The egg then moves through the fallopian tube.
The uterus, or womb, is a pear-shaped, muscular organ. The fallopian tubes lead into the uterus. If sperm cells are present in the fallopian tube, the egg cell may unite with a sperm cell. This process is called fertilization. The uterus is a hollow muscular organ, which is lined with a mucosa covered with ciliated epithelium, containing many uterine glands with the large end uppermost. The lower end tapers and projects into the vagina, where the uterus empties. The inner surface of the uterus has a spongy tissue that nourishes the fertilized egg as it develops. The outer walls of the uterus are highly elastic to allow for the expansion. If an egg is not fertilized, it usually breaks up before reaching the uterus.
The vagina is a tubular passage lined with mucous membrane and extending from the external orifice to the uterus. The external opening of the vagina is between the urethral opening and the anus. Folds of flesh, the labia, enclose both the urethral and vaginal orifices. The entrance of the vagina is partly covered by a thin membrane, the hymen. The passage from the uterus leading out of the body is called the birth canal.
Sex arises in the higher organisms when two highly specialized cells, the germ cells called sperm (male) and ovum or egg (female), develop. The union of an egg with a sperm is sexual reproduction. All animals, with the exception of the elementary forms of life, reproduce sexually. Such reproduction is a normal physiological function, as is digestion, breathing, and circulation.
Biologically, sex is for procreation to continue the species. It is an expression of love between a man and a woman and binds them closer together. It is a biological merging of two personalities into one, a spiritual as well as a physical union. Conception is the fertilization of an ovum by a sperm, marking the onset of pregnancy from sexual intercourse.
Fertilization takes place in the mother's fallopian tubes. Three conditions are necessary for this fertilization to occur. First, an egg must be present in the fallopian tube. The egg is in the fallopian tube for approximately two days after ovulation. Second, sperm must also be in the fallopian tube. Sperm swim into the fallopian tube after they have been deposited in the woman's vagina.
The third event that leads to fertilization is the joining of the egg and the sperm. Millions of sperm surround the thin wall of tissue that covers the egg, trying to break through to the egg. Only one sperm can succeed. If one sperm enters the egg, fertilization occurs. Immediately, the fertilized cell begins to grow. The egg splits into two cells. These two cells divide into four, these four become eight, these eight become sixteen, and so on. What starts as one fertilized cell grows in nine months into a baby made of 200 billion cells.
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