Our sex is determined by chromosomes. A man has an XY chromosome and a woman has an XX chromosome. There are other combinations (involving three chromosomes) but the Y chromosome is associated with being male. A boy inherits a Y chromosome from his father. The father’s genes (not the mother’s) determine the sex of a baby. Every foetus has a pair of gonads. The sex chromosome determines whether they develop into ovaries or testes, which produce hormones and gametes (sperm or egg cell).
A man’s internal anatomy includes the penis, testes, epididymis, seminal vesicles and prostate gland. The testes (inside the testicles) are the male gonads. They are 4 to 5 cm long and 2.5 cm in diameter. The testes produce male gametes (sperm), containing genetic material. Sperm are produced regularly from the testes throughout a man’s life (called spermatogenesis). A man’s sperm are tiny (relative to the size of the egg) and mobile. There are millions of them (versus only one egg) so they are more dispensable. Sperm cover some distance to reach the egg. The testes are also endocrine glands, which produce testosterone. The testes are located below the penis within a pouch of skin (a sac) called the scrotum. The testes are outside the body and so cooler than body temperature, which is necessary for sperm development. The testes are suspended by the spermatic cords and are highly vulnerable.
The epididymis is a system of ducts that receive immature sperm from the testes. Its function is to develop immature sperm and to store mature sperm. The vas deferens are two more fibrous and muscular tubes through which sperm pass from the epididymis to the urethra. Seminal fluid is produced during ejaculation by the seminal vesicles, the prostate and Cowper’s (or bulbourethral) gland. Nutrients in the fluid keep the sperm alive and mobile.
There are two seminal vesicles. Each 7.5 cm sac is lined with a mucous membrane, which secretes a pale fluid (containing sugars) to provide energy for sperm cells. This fluid makes up about two-thirds of semen. Tubes leading from the seminal vesicles, join the vas deferens to form the ejaculatory duct. Each ejaculatory duct empties into the urethra. The prostate is a firm muscular gland about the size of a chestnut, located near the internal opening of the urethra within the pelvic cavity. During ejaculation, the prostate produces a milky, alkaline fluid containing enzymes that increase sperm motility. Cowper’s glands are located at the base of the penis. During sexual activity, these glands secrete an alkaline fluid which helps neutralize acidity from urine in the urethra and acidity in the vagina.
A woman’s internal anatomy includes organs and structures that promote the production, support, growth, and development of female gametes (egg cells) and a growing foetus. The ovaries are gonads, which produce female gametes (ova or eggs). Ovaries are also endocrine glands, producing the hormones progesterone and oestrogen, which are sex hormones that govern early development and contribute to the menstrual cycle. There is one ovary on each side of the womb. Each ovary is about 4 cm long and 2 cm wide. The ovarian ligament attaches it to the uterus while a suspensory ligament attaches it to the pelvic wall. A woman’s genetic material (called ova) contains nutrients for the foetus and so is relatively large. These eggs are released one at a time from the ovaries on a regular monthly schedule from puberty until just after a woman’s last period (called the menopause) in late middle age.
During ovulation, an ovum is dispensed from an ovary’s surface. The Fallopian tubes transport the egg cell from the ovaries to the uterus (womb). Fertilization typically occurs in these tubes. The fallopian tube, or oviduct, is about 10 cm long. Once released, ova travel through the fallopian tube to the womb. The fallopian tubes lead into the upper part of the womb, one on either side, while the lower part of the uterus leads down into the vagina.
Also called the womb, the uterus is where a developing foetus resides during pregnancy. The uterus is a hollow, thick-walled, pear-shaped muscular internal organ (a duct) that is located in the pelvic cavity between the bladder and the rectum. The cervix is the lowest part of the uterus and leads to the vagina. Both the cervix and the vagina are internal reproductive organs that are largely without nerves. A woman cannot easily identify the anatomy stimulated by intercourse because the sensations are diffuse. Nevertheless, women who claim to orgasm from intercourse say this anatomy is the origin of their orgasms. Ligaments hold the upper part of the uterus in suspension; the lower part is embedded in fibrous tissue. When an egg is fertilized, it embeds itself into the uterine wall and develops. During pregnancy the uterus expands. After childbirth, the uterus shrinks back to its normal size.
The vagina is a fibrous, muscular canal leading from the cervix (opening of the uterus) to the external portion of the genital canal. It is part of the birth canal. The mouth and vagina have characteristics in common. Both are susceptible to thrush (a fungal infection), being warm and moist. Thrush causes an itchy feeling in the vagina. Women may assume that these infections arise because they have done something improper. This is not the case. Women should wash between their legs thoroughly at least once a day as well as after defecating. Cystitis, an infection of the bladder, causes a stinging sensation in the bladder and is accompanied by an urgent need to urinate. Medication can alleviate the symptoms and eliminate the infection.
There is a great deal of anatomic and clinical evidence that most of the interior of the vagina is without nerves. A considerable amount of surgery may be performed inside the vagina without need for anesthetics. (Alfred Kinsey)