Based on what survives, clinical anatomists believe that Leonardo's anatomical work was hundreds of years ahead of its time. He drew directly from dissections and made his drawings illegally at first; then with special permission from the Catholic church for the purposes of being used in a medical textbook. (**In 1990. an American doctor, Dr Frank Meshberger, published a report in the Journal of the American Medical Association in which he suggested that a cross-section image of a brain was concealed in the Sistine chapel panel of the Creation of Adam.
Our biological sex is how we are defined as female or male
or intersex. It describes our internal and external bodies — including our
sexual and reproductive anatomy, our genetic makeup, and our hormones. Our gender is our biological, social, and legal status as
girls and boys, women and men, and transgender.
When a child is born with external sex organs and/or sex chromosomes that are not clearly distinguishable as female or male, this is called intersex (1 in 2,000 people born in the U.S. is intersex). When a person's gender identity strongly conflicts with his/her biological sex — he/she is referred to as transgender. Some intersex people are transgender, but intersex does not necessarily mean transgender, and transgender does not necessarily mean intersex.
Our biological sex is established when an egg is fertilized. Most often, men ejaculate two types of sperm. One type has X chromosomes and the other type has Y chromosomes. When sperm fertilizes an egg, its X or Y chromosome combines with the X chromosome of the egg.
A girl has XX chromosomes and female sex and reproductive organs. When she reaches puberty, she will produce hormones that will cause her to develop into a woman.
A boy has XY chromosomes and male sex and reproductive organs. At puberty, he will produce hormones that will cause him to develop into a man.
Sometimes, a child is born with sex chromosomes that are different from the usual XX of the female or the XY of the male. Two common chromosomal intersex conditions are: Turner Syndrome = XO and Klienfelter’s Syndrome = XXY. The child may develop sex and/or reproductive organs that are ambiguous — not completely female and not completely male. Ambiguous sex organs may also be caused by other reasons and not be identified until puberty. Some people live all their lives without ever knowing they have an intersex condition, because the differences the person has cannot be found without testing chromosomes and hormones, or examining internal sex organs.
The penis is a man’s reproductive and sex organ. It is formed of three columns of spongy tissue — the corpus spongiosum and two corpora cavernosa — that fill with blood during sexual excitement, causing an erection (“hard on”). The penis extends from the lower portion of the belly. It is made up of a shaft and a glans (also known as the head) and is very sensitive to the touch. A man’s urethra is enclosed in his penis. It carries urine, pre-ejaculate, and semen out of his body.
The shaft looks like a tube. The shaft of the penis is about 1–3 inches long when soft. During an erection, the shaft expands to generally reach 4–6 inches.
The glans is the soft and highly sensitive part of the penis, located at its tip.
Opening of the Urethra
The opening of the urethra is located at the tip of the penis. This is where pre-ejaculate, semen, and urine leave the body.
The foreskin is a retractable tube of skin that covers and protects the head (glans). Some men have had their foreskin removed by circumcision during infancy. Some choose to be circumcised later in life.
The frenulum is where the foreskin attaches to the underside of the penis just below the glans. Usually, a portion of it remains after circumcision.
The scrotum is a sac of skin divided into two parts, enclosing the internal reproductive organs — the testicles.
The testicles are two ball-like glands inside the scrotum that produce sperm and hormones, including testosterone. Also called testes, the testicles are sensitive to the touch.
The epididymis is the tube in which sperm mature. An epididymis leads from each testicle to each vas deferens. It stores sperm before ejaculation. It is tightly coiled on top of and behind each testicle.
The muscle that automatically brings the testicles closer to the body as temperatures get colder or when the front or inner surface of the thigh is stimulated. The automatic response of the cremaster muscle is called the cremaster reflex. (If only one thigh is stimulated, only the testicle closest to the stimulated thigh is elevated.)
A vas deferens is a long, narrow tube that carries sperm from the epididymis to the seminal vesicles during ejaculation. There are two of them — one connected to each epididymis.
The prostate gland produces a fluid that helps sperm move through a man’s reproductive tract. The prostate gland is about the size of a walnut or golf ball. The prostate is sensitive to pressure and to the touch — “the male G-spot.”
The Cowper’s glands are beneath the prostate and attach to the urethra. They produce a fluid — pre-ejaculate or pre-cum — that prepares the urethra for ejaculation. Pre-ejaculate reduces friction in the urethra, making it easier for semen to pass through. Cowper’s glands are also called bulbourethral glands.
Seminal vesicles are two small organs that produce seminal fluid. The seminal vesicles are located below the bladder.
The urethra is a tube that empties the bladder and carries urine, pre-ejaculate, and semen to the urethral opening.