Human Sexuality

I

Introduction

Human Sexuality, general term referring to various sexually related aspects of human life, including physical and psychological development, and behaviors, attitudes, and social customs associated with the individual's sense of gender, relationships, sexual activity, mate selection, and reproduction. Sexuality permeates many areas of human life and culture, thereby setting humans apart from other members of the animal kingdom, in which the objective of sexuality is more often confined to reproduction. This article discusses the sexual anatomy, development, physiology, and behavior of human beings. For a more general discussion of animal reproduction, see Sex and Reproduction.

II

Human Sexual Characteristics

Sexual characteristics are divided into two types. Primary sexual characteristics are directly related to reproduction and include the sex organs (genitalia). Secondary sexual characteristics are attributes other than the sex organs that generally distinguish one sex from the other but are not essential to reproduction, such as the larger breasts characteristic of women and the facial hair and deeper voices characteristic of men.

A

Female Sexual Organs

Primary sexual characteristics of women include the external genitalia (vulva) and the internal organs that make it possible for a woman to produce ova (eggs) and become pregnant. The vulva includes the mons pubis, the most visible part of the woman's external genitalia, which is the pad of fatty tissue that covers the pubic bone and is commonly covered by pubic hair; the labia majora, the large outer lips; and the labia minora, the smaller, hairless inner lips that run along the edge of the vaginal opening and often fold over to cover it. The labia minora come together in front to form the clitoral hood, which covers the clitoris, a sensitive organ that is very important to the woman's sexual response. The opening of the urethra, the tubular vessel through which urine passes, is located midway between the clitoris and the vaginal opening. The area where the labia majora join behind the vagina is called the fourchette. The area of skin between the vaginal opening and the anus is the perineum. The hymen is a thin membrane that partially covers the vaginal opening. If the hymen is extensive and is still present at first intercourse, it may be broken or stretched as the penis enters the vagina and some bleeding and pain may occur, although more typically its presence is unnoticed. The presence or absence of a hymen is not a reliable indicator of virginity, although historically it was viewed as such.

The internal sex organs of the female consist of the vagina, uterus, fallopian tubes (or oviducts), and ovaries. The vagina is a flexible tube-shaped organ that is the passageway between the uterus and the opening in the vulva. Because during birth the baby travels from the uterus through the vagina, the vagina is also known as the birth canal. The woman's menstrual flow comes out of the uterus and through the vagina. When a man and a woman engage in vaginal intercourse, the penis is inserted into the vagina.

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The cervix is located at the bottom of the uterus and includes the opening between the vagina and the uterus. The uterus is a muscular organ that has an inner lining (endometrium) richly supplied with blood vessels and glands. During pregnancy, the uterus holds and nourishes the developing fetus. Although the uterus is normally about the size of a fist, during pregnancy it is capable of stretching to accommodate a fully developed fetus, which is typically about 50 cm (about 20 in) long and weighs about 3.5 kg (about 7.5 lbs). The uterine muscles also produce the strong contractions of labor.

At the top of the uterus are the pair of fallopian tubes that lead to the ovaries. The two ovaries produce eggs, or ova (the female sex cells that can become fertilized), and female sex hormones, primarily estrogens and progesterone. The fallopian tubes have fingerlike projections at the ends near the ovaries that sweep the egg into the fallopian tube after it is released from the ovaries. If sperm are present in the fallopian tube, fertilization (conception) may occur and the fertilized egg will be swept into the uterus by cilia (hairlike projections inside the fallopian tube).

B

Male Sexual Organs

The external sex organs of men are the penis and the scrotum. The penis is a sensitive organ important to reproduction and urination and to sexual pleasure. At its tip is the glans, which contains the urethral opening, through which urine passes. The ridge that separates the glans from the body of the penis is called the corona (Latin for “crown”), or coronal ridge. The glans and the corona are the most sensitive parts of the penis. The glans is covered with a foreskin (prepuce) unless the man has been circumcised, in which case the foreskin has been surgically removed.

The penis contains three cylinders of tissue that run parallel to the urethra. During sexual arousal, these tissues become engorged with blood and expand, causing the penis to enlarge and become erect (erection or tumescence). Men do not have a penis bone or a muscle that causes erection, as do some other animals.

The scrotum is a pouch that hangs below the penis and contains the two testes, which produce sperm (the male sex cell responsible for fertilization) and are considered part of the internal genitalia. The testes also are the primary producers of testosterone (male sex hormone) in men. Inside the testes are about 1,000 seminiferous tubules that manufacture and store the sperm. The scrotum can pull up closer to the body when the surrounding temperature is low and can drop farther away when the temperature is hot in order to keep the testes at an optimal, constant temperature somewhat lower than body temperature.

After sperm are produced, they move out of each testis and into the epididymis, a long tube coiled against the testis, where the sperm are stored and mature. The vas deferens transports the sperm from the epididymis through the prostate, after which the vas deferens becomes the ejaculatory duct. Here, fluids from the prostate and seminal vesicles (small sacs that hold semen) combine with the sperm to form semen, a thick, yellowish-white fluid. The average discharge of semen, called ejaculate, contains approximately 300 million sperm.