Wednesday, June 26, 2019
Home Biological aspects of sexuality Anatomy & development Human sexual anatomy and development

Human sexual anatomy and development

Sexuality is about appreciating the human body and sexual anatomy. We should understand the function of the male and female genitalia as well as the changes that occur throughout our lives. It is also useful to know what kind of stimulation may be pleasurable or is involved in achieving orgasm.

The step of evolving two sexes was inevitable because of the huge advantage that sexual reproduction provides. Sexual reproduction involves combining gene pools from two parents and allows more rapid development of genetic diversity. Lifeforms can adapt more quickly to changing environments. Those individuals with a more advantageous set of genes reproduce and survive more successfully on average than those who are less advantaged. Many organisms reproduce by using this method, called natural selection.

There is a specific chromosome that determines our sex. 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 (rather than the mother’s) determine the sex of a baby.

Every human embryo develops a pair of gonads. The sex chromosomes determine whether the gonads in the embryo become the testes (for a boy) or the ovaries (for a girl). The gonads become centres for the production of genetic material (eggs or spermatozoa). The ovaries store the eggs, which are held until they mature and then released one at a time during a woman’s monthly cycle. Once the testes of a male foetus start producing androgens (primarily male hormones such as testosterone) they drive further change, which is concluded at puberty. This process is called sexual differentiation.

Sometimes we have only one body part. We have only one head and one liver. Sometimes we have two for backup. We have two legs and two kidneys. There are relatively few anatomical differences between the sexes. A man has nipples but he never develops the mammary glands that form the female breast. The key differences relate to sexual anatomy but even here there is symmetry. A woman has two ovaries and a man has two testes.

We can see the evidence for how orgasm evolved by looking at the development of the human foetus. Unlike the internal reproductive anatomy, the phallus is not an either-or option. Everyone has a sex organ and so everyone (to varying degrees) is potentially capable of orgasm. Every foetus has a rudimentary phallus. This is called the genital tubercle. It is impossible to determine the sex of a foetus until after 14 weeks. Only the extent of the development of the phallus varies depending on hormones sent out by the testes (or not). The genital tubercle undergoes significantly more development in a boy so the penis is more complex than the clitoris.

We all achieve orgasm through the same mechanism regardless of gender and orientation. Women’s genitalia (clitoris and labia) are anatomically equivalent to men’s genitalia (penis and testicles) and are similarly positioned. The labioscrotal folds of the embryo become the scrotum (for a boy) and the labia (for a girl). As a woman ages, the parallel with the male sexual anatomy becomes much more apparent as the skin either side of her labia majora looks increasingly like the wrinkly hairy skin of the testicles.

Orgasm is a primitive response of the human body that has evolved in both sexes via an identical mechanism. The phallus is the only organ that is involved in orgasmic response. The penis and clitoris are easily reached by the hands so that self-stimulation (called masturbation) is possible. This enables a responsive individual to supply their own stimulation if desired.

Male responsiveness is required for reproduction. Male orgasm triggers the ejaculation of semen (which contains) sperm. Most men experience orgasm regularly. Most young men especially are capable of frequent ejaculation. Intercourse is an act that is intended to facilitate male orgasm. The associated pleasures of intercourse make it a highly desired act for men.

Female responsiveness has no biological justification. Female orgasm occurs sporadically and infrequently. Very few women are responsive. Female responsiveness is evidenced by a woman’s solitary masturbatory activity. A responsive woman generates her arousal consciously by focusing on eroticism. As a result of clitoral tumescence (increased blood-flow), she instinctively stimulates the internal clitoral organ and thrusts to orgasm.

Girls and boys have equal amounts of androgens until the age of 7 or 8. At puberty levels of androgens rise markedly in boys but less so in girls. Women have around two thirds of men’s levels of androgens, including testosterone (involved in libido and sexual response). Boy and girls also have equal amounts of estrogens. At puberty the levels of estrogens increase abruptly in girls but only slightly in boys. Women’s levels of estrogens are much higher than men’s. There is a steep increase in male responsiveness at adolescence but a much lower increase in female responsiveness.

… the reported levels of the 17-ketosteroids in the human male differ from the reported levels of the 17-ketosteroids in the human female in a manner which more or less parallels the differences which we have found between the median frequencies of orgasm at various ages in the two sexes. (Alfred Kinsey 1953)