The Blind Watchmaker: Why the Evidence of Evolution Reveals a Universe without Design is a 1986 book by Richard Dawkins, in which the author presents an explanation of, and argument for, the theory of evolution by means of natural selection. He also presents arguments to refute certain criticisms made of his first book, The Selfish Gene. (Both books espouse the gene-centric view of evolution.) An unabridged audiobook edition was released in 2011, narrated by Richard Dawkins and Lalla Ward.
Synopsis
"Biomorph"
that randomly evolves following changes of several numeric "genes",
determining its shape; the gene values are given as bars on the top
The title of the book refers to the watchmaker analogy made famous by William Paley in his 1802 book Natural Theology or Evidences of the Existence and Attributes of the Deity. Paley, writing long before Charles Darwin published On the Origin of Species in 1859, held that the complexity of living organisms was evidence of the existence of a divine creator
by drawing a parallel with the way in which the existence of a watch
compels belief in an intelligent watchmaker. Dawkins, in contrasting the
differences between human design and its potential for planning with
the workings of natural selection, therefore dubbed evolutionary
processes as analogous to a blind watchmaker.
To dispel the idea that complexity cannot arise without the
intervention of a "creator", Dawkins uses the example of the eye.
Beginning with a simple organism, capable only of distinguishing between
light and dark, in only the crudest fashion, he takes the reader
through a series of minor modifications, which build in sophistication
until we arrive at the elegant and complex mammalian eye. In making this
journey, he points to several creatures whose various seeing apparatus
are, whilst still useful, living examples of intermediate levels of
complexity.
In developing his argument that natural selection can explain the
complex adaptations of organisms, Dawkins' first concern is to
illustrate the difference between the potential for the development of
complexity as a result of pure randomness, as opposed to that of
randomness coupled with cumulative selection. He demonstrates this by
the example of the weasel program. Dawkins then describes his experiences with a more sophisticated computer simulation of artificial selection implemented in a program also called The Blind Watchmaker, which was sold separately as a teaching aid.
The program displayed a two-dimensional shape (a "biomorph") made
up of straight black lines, the length, position, and angle of which
were defined by a simple set of rules and instructions (analogous to a
genome). Adding new lines (or removing them) based on these rules
offered a discrete set of possible new shapes (mutations), which were
displayed on screen so that the user could choose between them. The
chosen mutation would then be the basis for another generation of
biomorph mutants to be chosen from, and so on. Thus, the user, by
selection, could steer the evolution of biomorphs. This process often
produced images which were reminiscent of real organisms, for instance beetles, bats, or trees.
Dawkins speculated that the unnatural selection role played by the user
in this program could be replaced by a more natural agent if, for
example, colourful biomorphs could be selected by butterflies or other
insects, via a touch-sensitive display set up in a garden.
In an appendix to the 1996 edition of the book, Dawkins explains
how his experiences with computer models led him to a greater
appreciation of the role of embryological
constraints on natural selection. In particular, he recognised that
certain patterns of embryological development could lead to the success
of a related group of species in filling varied ecological niches, though he emphasised that this should not be confused with group selection. He dubbed this insight the evolution of evolvability.
After arguing that evolution is capable of explaining the origin
of complexity, near the end of the book Dawkins uses this to argue
against the existence of God: "a deity capable of engineering all the
organized complexity in the world, either instantaneously or by guiding
evolution ... must already have been vastly complex in the first place
..." He calls this "postulating organized complexity without offering an
explanation".
In the preface, Dawkins states that he wrote the book "to persuade the reader, not just that the Darwinian world-view happens to be true, but that it is the only known theory that could, in principle, solve the mystery of our existence".
Reception
Tim Radford, writing in The Guardian,
noted that despite Dawkins's "combative secular humanism", he had
written "a patient, often beautiful book... that begins in a generous
mood and sustains its generosity to the end." 30 years on, people still
read the book, Radford argues, because it is "one of the best books ever
to address, patiently and persuasively, the question that has baffled
bishops and disconcerted dissenters alike: how did nature achieve its
astonishing complexity and variety?"
Philosopher and historian of biology Michael T. Ghiselin, writing in The New York Times,
comments that Dawkins "succeeds admirably in showing how natural
selection allows biologists to dispense with such notions as purpose and
design". He notes that analogies with computer programs have their
limitations, but are still useful. Ghiselin observes that Dawkins is "not content with rebutting creationists" but goes on to press home his arguments against alternative theories to neo-Darwinism.
He thinks the book fills the need to know more about evolution that
creationists "would conceal from them." He concludes that "Readers who
are not outraged will be delighted."
The American philosopher of religion Dallas Willard,
reflecting on the book, denies the connection of evolution to the
validity of arguments from design to God: whereas, he asserts, Dawkins
seems to consider the arguments to rest entirely on that basis. Willard
argues that Chapter 6, "Origins and Miracles", attempts the "hard task"
of making not just a blind watchmaker but "a blind watchmaker
watchmaker", which he comments would have made an "honest" title for the
book. He notes that Dawkins demolishes several "weak" arguments, such
as the argument from personal incredulity. He denies that Dawkins's
computer "exercises" and arguments from gradual change show that complex
forms of life could have evolved. Willard concludes by arguing that in
writing this book, Dawkins is not functioning as a scientist "in the
line of Darwin", but as "just a naturalist metaphysician".
Influence
The engineer Theo Jansen read the book in 1986 and became fascinated by evolution and natural selection. Since 1990 he has been building kinetic sculptures, the Strandbeest, capable of walking when impelled by the wind.
The fish family Syngnathidae has the unique characteristic of a highly derived form of male brood care referred to as "male pregnancy". The family is highly diverse, containing around 300 different species of fish. Included in Syngnathidae are seahorses, the pipefish, and the weedy and leafy seadragons. The males of some of these species possess a brood pouch on the trunk or tail; in other species, the eggs
are merely attached to the male's trunk or tail when the female lays
them. Although biologists' definitions of pregnancy somewhat differ, all
members of the family are considered by ichthyologists to display male pregnancy, even those without an external brood pouch.
Fertilization may take place in the pouch or in the water before
implantation, but in either case, syngnathids' male pregnancy ensures
them complete confidence of paternity. After implantation in or on the brood pouch or brood patch, the male incubates the eggs. Many species osmoregulate the brood pouch fluid to maintain a suitable pH
level for the developing embryos. In at least some species, the male
also provisions his offspring with nutrients such as glucose and amino
acids through the highly vascularized attachment sites in or on his
body.
This period of incubation can take much longer than the
production of another clutch of eggs by the female, especially in
temperate regions where pregnancies last longer, leading to a reproductive environment in which sexual selection
can be stronger on females than on males due to increased male parental
investment. This reversal of usual sex roles has only been found in
pipefishes, whereas seahorses have largely been accepted as monogamous. Some pipefish species display classical polyandry
because of this unique situation. Male syngnathids usually prefer
females with large body size and prominent ornaments such as blue skin
pigmentation or skin folds. Syngnathid males in some species are
apparently capable of absorbing eggs or embryos while in the brood
pouch. In these cases, embryos with the highest survival rate are those whose mothers display the preferred phenotype.
Syngnathidae is the only family in the animal kingdom to which the term "male pregnancy" has been applied.
Other animals
In 2021, Chinese researchers at the Naval Medical University in Shanghai published a preprint of a study that attempted to impregnate male rats, using parabiosis
with female rats. Each male rat was castrated, surgically joined to a
female, and given a transplanted uterus. The researchers then implanted
embryos in both the uterus of the male and female parabionts.
Pregnancies were allowed to develop until two days before the end of a
full term, and were terminated by caesarean section.
The researchers studied 46 such parabiotic pairs. In over half of the
pairs, neither the male nor female became pregnant with normal embryos;
in about one-third of the pairs, only the female became thus pregnant;
and in six pairs, both the female and male became pregnant. There were
no pairs in which only the male parabiont rat became pregnant. The study attracted much attention and controversy, with some
researchers questioning the utility of such research, and others raising
questions of bioethics issues.
In 2023, Japanese scientists used skin cells from 2 male mice to create eggs and fathered a litter of seven babies. The eggs were implanted in surrogate female mice. The current downside is the success rate is 1% (7 mice were born out of 630 attempts). This milestone in reproductive biology was published in British scientific journal Nature, cultivating the idea of more reproductive possibilities in the future. The team was led by developmental biologist Katsuhiko Hayashi of the Osaka and Kyushu universities.
Mammalian males, including humans, do not possess a uterus to gestate offspring. The theoretical issue of male ectopic pregnancy (pregnancy outside the uterine cavity) by surgical implantation has been addressed by experts in the field of fertility medicine,
who stress that the concept of ectopic implantation, while
theoretically plausible, has never been attempted and would be difficult
to justify – even for a woman lacking a uterus – owing to the extreme
health risks to both the parent and child.
Robert Winston, a pioneer of in-vitro fertilization, told London's Sunday Times
that "male pregnancy would certainly be possible" by having an embryo
implanted in a man's abdomen – with the placenta attached to an internal
organ such as the bowel – and later delivered surgically.Ectopic implantation of the embryo along the abdominal wall, and resulting placenta growth would, however, be very dangerous and potentially fatal for the host, and is therefore unlikely to be studied in humans. Gillian Lockwood, medical director of Midland Fertility Services, a
British fertility clinic, noted that the abdomen has not evolved to
separate from the placenta during delivery, hence the danger of an
ectopic pregnancy. Bioethicist Glenn McGee said "the question is not 'Can a man do it?'. It's 'If a man does have a successful [ectopic] pregnancy, can he survive it?'"
Since 2000, several hoax web sites have appeared on the Internet purporting to describe the world's first pregnant man. While some rely
on legitimate scientific claims, no such experiment has ever been
reported. Fertility clinician Cecil Jacobson claimed to have transplanted a fertilized egg from a female baboon to the omentum in the abdominal cavity of a male baboon
in the mid-1960s, which then carried the fetus for four months;
however, Jacobson did not publish his claims in a scientific journal,
and was subsequently convicted on several unrelated counts of fraud for
ethical misconduct.
Transplanting a uterus into a male body poses a challenge due to the
lack of natural ligaments, vasculature, and hormones required to support
the uterus. The uterus would either have to be donated by a willing
donor or be tissue-engineered using the male's stem cells and then implanted into the pelvic region. Afterward, an in vitro fertilisation (IVF) procedure would be followed to insert the embryo into the male's transplanted womb.
In 1931, transgender female Lili Elbe (assigned male at birth) underwent a uterus transplant in an attempt to achieve pregnancy, but died of complications following the procedure.
Pregnancy in transgender men
Pregnant trans man Zack Elías and his transgender wife, Diane Rodríguez.
Hindu religious texts, such as the Mahabharata (composition range from 300 BCE to 400 CE) and the Vishnu Purana
(composition range from 400 BCE to 900 CE), contain the story of King
Yuvanashva, who accidentally becomes pregnant by drinking a sacred
potion intended for his queens. He proceeds with the pregnancy and his
son, Mandhata, goes on to become a king renowned for his greatness, benevolence, and generosity.
In ancient Greek religion and myth, a version of Dionysus' birth (known in mainland Greece after 500 BCE, popularized in 500 CE), claims that Dionysus was sewn into Zeus’ thigh. "So the rounded thigh
in labour became female, and the boy too soon born was brought forth,
but not in a mother's way, having passed from a mother's womb to a
father's." Athena was believed to have been born from the forehead of her father Zeus. The earliest mention is in Book V of the Iliad (c. 800 BCE), when Ares accuses Zeus of being biased in favor of Athena because "autos egeinao" (literally "you fathered her", but probably intended as "you gave birth to her").
Books
In the 2nd-century novel A True Story by Lucian, there are no women on the moon, and as such boys below 25 are considered wives bearing children in their calves. Some modern science fiction writers have picked up on the concept of male pregnancy in various ways.
The 16th century novel Journey to the West, described as arguably the most popular literary work in East Asia, contains an episode in which two of the men in the main cast
accidentally become pregnant by drinking from the “Child-and-Mother
River” (Zimu he,子母河), then abort the pregnancies by travelling to the
Abortion Spring (Luo tai quan, 落胎泉).
Ursula K. Le Guin's novel The Left Hand of Darkness
(1969) contains the sentence "The king was pregnant", and explores a
society in which pregnancy can be experienced by anyone, since
individuals are not sexually differentiated during most of their life
and can become capable of inseminating or gestating at different times.
Larry Niven's 1969 essay "Man of Steel, Woman of Kleenex"
ends with considering Superman as a carrier for his own baby, due to
the difficulties a human female might encounter carrying a superpowered
fetus.
The concept of male pregnancy has been the subject of popular films, generally as a comedic device.
The 1978 comedy film Rabbit Test stars Billy Crystal as a young man who inexplicably becomes pregnant instead of his female sex partner.
The 1990 BBC television comedy drama Frankenstein's Baby features a Dr. Eva Frankenstein helping a male patient to become the “world's first” pregnant man.
The 1994 science fiction comedy/drama Junior stars Arnold Schwarzenegger as a fertility researcher who experiments on himself; the screenplay was inspired by a 1985 article in Omni magazine.
The 2015 romantic comedy Paternity Leave explores the concept of a gay couple surprised with a miraculous pregnancy in feature length.
The 2017 film Mamaboy
stars Sean O'Donnell as a teenager who decides to undergo an
experimental procedure that enables him to carry his girlfriend's baby
to term.
In 2019, as a social commentary on the issue of abortion, The Blacklist had an episode which had anti-abortion
men being kidnapped and forced to be pregnant. One stayed consistent
with their belief and gave birth while the other became hypocritical and
sought to get an abortion despite it being illegal in their state.
Horror rarely dips into male pregnancy in depth. Minor appearances exist in the well-known Alien series, in which the first chestburster
appears, as a result of the host organism using human bodies to gestate
its young. While this concept is repeated and parodied widely, the
origin is as much sci-fi as it is horror. In the 2019 anthology film The Mortuary Collection,
a predatory fraternity brother named Jake has sex with a woman, using
stealthing to trick her into having sex with him without a condom. As a
result, her young grows in him rapidly over the course of a day,
resulting in his bloody death when the child emerges. In 2020, the
horror film Amulet
depicts a soldier returning from war to live in a claustrophobic house
with a woman and her mother, and a dark presence that may be lurking
there as well. Like the former movie, male pregnancy serves as a sort of
punishment for a man's sins.
Television
The concept appears frequently as a comedic gag in numerous television programs as well.
In a 1981 episode of the Canadian sketch comedy series Bizarre, the show's resident daredevil character Super Dave Osborne (Bob Einstein) performs, as one of his many stunts, carrying and giving birth to a baby.
In the BBC science fiction comedy series Red Dwarf, the main character Dave Lister becomes pregnant after having sex with a female version of himself in an alternate universe.
In an episode of Sliders,
the quartet "slides" into an alternate world in which babies develop
during their final months in the father because a worldwide disease has
kept women from being able to carry children beyond their first
trimester.
In the popular fantasy series Charmed's
fifth season, during a dream spell gone wrong, Leo ends up pregnant
with Piper's baby for a good deal of the episode, leading to her
referring to him as an "incubator" and at times berating him for
"upsetting the baby".
The possibility of extraterrestrial life having different reproductive sexuality is the basis for many references. In the Star Trek: Enterprise episode "Unexpected", Trip Tucker becomes pregnant with the offspring of a female of another species, described late in the chapter, set in mid-22nd century, as being "the first recorded instance of a human male pregnancy".
In the video game The Sims 2 male characters can be impregnated via cheat codes or alien abduction. In the American Dad! episode "Deacon Stan, Jesus Man",
the boy Steve becomes impregnated after giving the mouth-to-mouth
resuscitation to the extraterrestrial Roger, then unwittingly passes it
on to his girlfriend via a kiss. In the animated series Futurama, the extraterrestrial Kif can be impregnated by a touch. In Doctor Who (series 11), an episode features an alien man going into labor. In the SciFi Channel miniseries, Farscape: The Peacekeeper Wars, the extraterrestrial Rygel becomes impregnated with human John and Aeryn's baby. In the series Alien Nation,
when Tectonese main character George Francisco and his wife Susan
decide to have a third child, it is revealed that, in order to conceive,
a Tectonese couple needs a third party, called a binnaum, to
complete impregnation, and that the male carries the baby—encased in a
pod—during the final months of gestation. In the animated series The Fairly OddParents in the TV film Fairly OddBaby, the fairy Cosmo was pregnant with Baby Poof. Additionally, Robert Sheckley's 1989 short story Love Song From the Stars also contains this element. My Friends from Afar,
a science fiction Singaporean drama series, Xiang Lin becomes pregnant
to his surprise by kissing Tianning and his pregnancy provides an
ongoing plotline in the latter half of the series. However, after he
gives birth, it turns out that his species gives birth to eggs, which
hatch, after some time, a child that appears to be roughly four years
old.
In the Ozzy & Drix
episode "Ozzy Jr.", Ozzy thinks he is having a baby but is actually a
parasite growing in his belly caused by an infection by Strepfinger.
In the Ren & Stimpy "Adult Party Cartoon" episode "Stimpy's Pregnant", Stimpy is thought to be pregnant but Mr. Horse finds out that he is actually constipated.
In the Bluey episode "Dad Baby", Bandit Heeler shows Bluey and Bingo how to use a baby carrier, and he pretends to be pregnant and giving birth.
In the Ben 10: Alien Force
episode "Save the Last Dance", it is revealed that Necrofriggians have
an ability to asexually reproduce once every 80 years, building a large
nest made of digested metal where their eggs will hatch and their
offspring will feed on the metal, first eating from the nest before they
instinctively feed on solar plasma until they mature and starts their
own separate lives. Due to the Necrofriggian reproduction cycle, Big
Chill overtook Ben's
personality to carry out the process, but Ben did not remember anything
he did as Big Chill during this cycle, like eating metal and having 14
babies, and he felt very embarrassed when Gwen, Kevin and Julie
explained, and Kevin's teasing and calling him "mommy" did not help.
In The Three Stooges
episode "Even as IOU" Curly accidentally swallows a Vitamin Z pill
meant for a horse. However, the error allows Curly to give birth to an
Equidae, which the Stooges crown as a winning race horse.
The manga series He's Expecting
takes place in the future where men are suddenly capable of becoming
pregnant, though it is only a 10% chance of happening. The series
explores the workplace prejudice that men and women experience and the
titular character's efforts to change public opinion once he himself
becomes pregnant. It was later adapted into a television series of the same name.
Other
Virgil Wong, a performance artist, created a hoax sitefeaturing a fictitious male pregnancy, claiming to detail the pregnancy of his friend Lee Mingwei.
In 2021, Unicode approved the "pregnant man" and "pregnant person" emojis in version 14.0, and added to Emoji 14.0. However, this came with some controversy, as some viewed it to be "absurd".
Female
(left) and male (right) adult human bodies photographed in ventral
(above) and dorsal (below) perspectives. Naturally-occurring pubic, body, and facial hair have been deliberately removed to show anatomy.
The human body is the entire structure of a human being. It is composed of many different types of cells that together create tissues and subsequently organs and then organ systems.
The study of the human body includes anatomy, physiology, histology and embryology. The body varies anatomically
in known ways. Physiology focuses on the systems and organs of the
human body and their functions. Many systems and mechanisms interact in
order to maintain homeostasis, with safe levels of substances such as sugar, iron, and oxygen in the blood.
The body is studied by health professionals, physiologists, anatomists, and artists to assist them in their work.
The adult male body is about 60% total body water content of some 42 litres (9.2 imp gal; 11 US gal). This is made up of about 19 litres (4.2 imp gal; 5.0 US gal) of extracellular fluid including about 3.2 litres (0.70 imp gal; 0.85 US gal) of blood plasma and about 8.4 litres (1.8 imp gal; 2.2 US gal) of interstitial fluid, and about 23 litres (5.1 imp gal; 6.1 US gal) of fluid inside cells. The content, acidity and composition of the water inside and outside cells is carefully maintained. The main electrolytes in body water outside cells are sodium and chloride, whereas within cells it is potassium and other phosphates.
The body contains trillions of cells, the fundamental unit of life. At maturity, there are roughly 30 trillion cells, and 38 trillion bacteria in the body, an estimate arrived at by totaling the cell numbers of all the organs of the body and cell types. The skin of the body is also host to billions of commensal organisms as well as immune cells. Not all parts of the body are made from cells. Cells sit in an extracellular matrix that consists of proteins such as collagen, surrounded by extracellular fluids.
Each of the cells of the human body experiences, on average, tens of thousands of DNA damages per day. These damages can block genome replication or genome transcription, and if they are not repaired or are repaired incorrectly, they may lead to mutations, or other genome alterations that threaten cell viability.
Cells in the body function because of DNA. DNA sits within the nucleus of a cell. Here, parts of DNA are copied and sent to the body of the cell via RNA. The RNA is then used to createproteins,
which form the basis for cells, their activity, and their products.
Proteins dictate cell function and gene expression, a cell is able to
self-regulate by the amount of proteins produced. However, not all cells have DNA; some cells such as mature red blood cells lose their nucleus as they mature.
Tissues
Diagram of the different types of soft tissue in the body
The body consists of many different types of tissue, defined as cells that act with a specialised function. The study of tissues is called histology and is often done with a microscope. The body consists of four main types of tissues. These are lining cells (epithelia), connective tissue, nerve tissue and muscle tissue.
Cells
Cells that line surfaces exposed to the outside world or gastrointestinal tract (epithelia) or internal cavities (endothelium) come in numerous shapes and forms – from single layers of flat cells, to cells with small beating hair-like cilia in the lungs, to column-like cells that line the stomach.
Endothelial cells are cells that line internal cavities including blood
vessels and glands. Lining cells regulate what can and cannot pass
through them, protect internal structures, and function as sensory
surfaces.
1905 diagram of the internal organs of the human body
Organs, structured collections of cells with a specific function, mostly sit within the body, with the exception of skin. Examples include the heart, lungs and liver. Many organs reside within cavities within the body. These cavities include the abdomen (which contains the stomach, for example) and pleura, which contains the lungs.
The heart is composed of two atria and two ventricles. The primary purpose of the atria is to allow uninterrupted venous blood flow to the heart during ventricular systole. This allows enough blood to get into the ventricles during atrial systole. Consequently, the atria allows a cardiac output roughly 75% greater than would be possible without them. The purpose of the ventricles is to pump blood to the lungs through the right ventricle and to the rest of the body through the left ventricle.
The heart has an electrical conduction system to control the contraction and relaxation of the muscles. It starts in the sinoatrial node traveling through the atria causing them to pump blood into the ventricles. It then travels to the atrioventricular node,
which makes the signal slow down slightly allowing the ventricles to
fill with blood before pumping it out and starting the cycle over again.
Gallstones is a common disease in which one or more stones form in the gallbladder or biliary tract. Most people are asymptomatic but if a stone blocks the biliary tract, it causes a gallbladder attack;
symptoms may include sudden pain in the upper right abdomen or center
of the abdomen. Nausea and vomiting may also occur. Typical treatment is
removal of the gallbladder through a procedure called a cholecystectomy. Having gallstones is a risk factor for gallbladder cancer, which, although quite uncommon, is rapidly fatal if not diagnosed early.
The circulatory system consists of the heart and blood vessels (arteries, veins and capillaries). The heart propels the circulation of the blood, which serves as a "transportation system" to transfer oxygen, fuel, nutrients, waste products, immune cells and signaling molecules (i.e. hormones) from one part of the body to another. Paths of blood circulation within the human body can be divided into two circuits: the pulmonary circuit, which pumps blood to the lungs to receive oxygen and leave carbon dioxide,
and the systemic circuit, which carries blood from the heart off to the
rest of the body. The blood consists of fluid that carries cells in the circulation, including some that move from tissue to blood vessels and back, as well as the spleen and bone marrow.
Digestion begins in the mouth, which chews food into smaller pieces for easier digestion. Then it is swallowed, and moves through the esophagus to the stomach. In the stomach, food is mixed with gastric acids to allow the extraction of nutrients. What is left is called chyme; this then moves into the small intestine, which absorbs the nutrients and water from the chyme. What remains passes on to the large intestine, where it is dried to form feces; these are then stored in the rectum until they are expelled through the anus.
The endocrine system consists of the principal endocrine glands: the pituitary, thyroid, adrenals, pancreas, parathyroids, and gonads, but nearly all organs and tissues produce specific endocrine hormones
as well. The endocrine hormones serve as signals from one body system
to another regarding an enormous array of conditions, resulting in
variety of changes of function.
The integumentary system consists of the covering of the body (the skin), including hair and nails as well as other functionally important structures such as the sweat glands and sebaceous glands.
The skin provides containment, structure, and protection for other
organs, and serves as a major sensory interface with the outside world.
The lymphatic system
extracts, transports and metabolizes lymph, the fluid found in between
cells. The lymphatic system is similar to the circulatory system in
terms of both its structure and its most basic function, to carry a body
fluid.
The nervous system is subject to many different diseases. In epilepsy, abnormal electrical activity in the brain can cause seizures. In multiple sclerosis, the immune system attacks the nerve linings, damaging the nerves' ability to transmit signals. Amyotrophic lateral sclerosis (ALS), also known as Lou Gehrig's disease, is a motor neuron disease which gradually reduces movement in patients. There are also many other diseases of the nervous system.
Female puberty generally occurs between the ages of 9 and 13 and is characterized by ovulation and menstruation; the growth of secondary sex characteristics, such as growth of pubic and underarm hair, breast, uterine and vaginal growth, widening hips and increased height and weight, also occur during puberty. Male puberty sees the further development of the penis and testicles.
The female inner sex organs are the two ovaries, their fallopian tubes, the uterus, and the cervix. At birth there are about 70,000 immature egg cells
that degenerate until at puberty there are around 40,000. No more egg
cells are produced. Hormones stimulate the beginning of menstruation,
and the ongoing menstrual cycles. The female external sex organs are the vulva (labia, clitoris, and vestibule).
The respiratory system consists of the nose, nasopharynx, trachea, and lungs. It brings oxygen from the air and excretes carbon dioxide and water back into the air. First, air is pulled through the trachea into the lungs by the diaphragm pushing down, which creates a vacuum. Air is briefly stored inside small sacs known as alveoli (sing.: alveolus) before being expelled from the lungs when the diaphragm contracts again. Each alveolus is surrounded by capillaries carrying deoxygenated blood, which absorbs oxygen out of the air and into the bloodstream.
For the respiratory system to function properly, there need to be
as few impediments as possible to the movement of air within the lungs.
Inflammation of the lungs and excess mucus are common sources of breathing difficulties. In asthma, the respiratory system is persistently inflamed, causing wheezing or shortness of breath. Pneumonia occurs through infection of the alveoli, and may be caused by tuberculosis. Emphysema, commonly a result of smoking, is caused by damage to connections between the alveoli.
The urinary system consists of the two kidneys, two ureters, bladder, and urethra. It removes waste materials from the blood through urine, which carries a variety of waste molecules and excess ions and water out of the body.
First, the kidneys filter the blood through their respective nephrons, removing waste products like urea, creatinine and maintaining the proper balance of electrolytes and turning the waste products into urine by combining them with water from the blood. The kidneys filter about 150 quarts (170 liters) of blood daily, but
most of it is returned to the blood stream with only 1-2 quarts (1-2
liters) ending up as urine, which passes from the kidneys through the ureters into the bladder.
The smooth muscles lining the ureter walls continuously tighten and relax through a process called peristalsis, forcing small amounts of urine into the bladder every 10–15 seconds.
The bladder is a hollow balloon shaped organ located in the pelvis. It stores urine until the brain signals it to relax the urinary sphincter and release the urine into the urethra starting urination. A normal bladder can hold up to 16 ounces (half a liter) for 3–5 hours comfortably.
Human anatomy is the study of the shape and form of the human body. The human body has four limbs (two arms and two legs), a head and a neck, which connect to the torso. The body's shape is determined by a strong skeleton made of bone and cartilage, surrounded by fat (adipose tissue), muscle, connective tissue, organs, and other structures. The spine at the back of the skeleton contains the flexible vertebral column, which surrounds the spinal cord, which is a collection of nerve fibres connecting the brain to the rest of the body. Nerves
connect the spinal cord and brain to the rest of the body. All major
bones, muscles, and nerves in the body are named, with the exception of anatomical variations such as sesamoid bones and accessory muscles.
Blood vessels carry blood throughout the body, which moves because of the beating of the heart. Venules and veins
collect blood low in oxygen from tissues throughout the body. These
collect in progressively larger veins until they reach the body's two
largest veins, the superior and inferior vena cava, which drain blood into the right side of the heart. From here, the blood is pumped into the lungs where it receives oxygen and drains back into the left side of the heart. From here, it is pumped into the body's largest artery, the aorta, and then progressively smaller arteries and arterioles until it reaches tissue. Here, blood passes from small arteries into capillaries, then small veins and the process begins again. Blood carries oxygen, waste products, and hormones from one place in the body to another. Blood is filtered at the kidneys and liver.
Height, weight, shape and other body proportions vary individually and with age and sex. Body shape is influenced by the distribution of bones, muscle and fat tissue.
Human physiology is the study of how the human body functions. This includes the mechanical, physical, bioelectrical, and biochemical functions of humans in good health, from organs to the cells of which they are composed. The human body consists of many interacting systems of organs. These interact to maintain homeostasis, keeping the body in a stable state with safe levels of substances such as sugar and oxygen in the blood.
Each system contributes to homeostasis, of itself, other systems,
and the entire body. Some combined systems are referred to by joint
names. For example, the nervous system and the endocrine system operate
together as the neuroendocrine system. The nervous system receives information from the body, and transmits this to the brain via nerve impulses and neurotransmitters. At the same time, the endocrine system releases hormones, such as to help regulate blood pressure
and volume. Together, these systems regulate the internal environment
of the body, maintaining blood flow, posture, energy supply,
temperature, and acid balance (pH).
Development of the human body is the process of growth to maturity.
The process begins with fertilisation, where an egg released from the ovary of a female is penetrated by sperm. The egg then lodges in the uterus, where an embryo and later fetus develop until birth.
Growth and development occur after birth, and include both physical and
psychological development, influenced by genetic, hormonal,
environmental and other factors. Development and growth continue
throughout life, through childhood, adolescence, and through adulthood to old age, and are referred to as the process of aging.
Health professionals
learn about the human body from illustrations, models, and
demonstrations. Medical and dental students in addition gain practical
experience, for example by dissection of cadavers. Human anatomy, physiology, and biochemistry are basic medical sciences, generally taught to medical students in their first year at medical school.
In Western societies, the contexts for depictions of the human body include information, art and pornography.
Information includes both science and education, such as anatomical
drawings. Any ambiguous image not easily fitting into one of these
categories may be misinterpreted, leading to disputes. The most contentious disputes are between fine art and erotic images,
which define the legal distinction of which images are permitted or
prohibited.
The study of human physiology began with Hippocrates in Ancient Greece, around 420 BCE, and with Aristotle (384–322 BCE) who applied critical thinking and emphasis on the relationship between structure and function. Galen (c. 129 – c. 216) was the first to use experiments to probe the body's functions. The term physiology was introduced by the French physician Jean Fernel (1497–1558). In the 17th century, William Harvey (1578–1657) described the circulatory system, pioneering the combination of close observation with careful experiment. In the 19th century, physiological knowledge began to accumulate at a rapid rate with the cell theory of Matthias Schleiden and Theodor Schwann in 1838, that organisms are made up of cells. Claude Bernard (1813–1878) created the concept of the milieu interieur (internal environment), which Walter Cannon (1871–1945) later said was regulated to a steady state in homeostasis. In the 20th century, the physiologists Knut Schmidt-Nielsen and George Bartholomew extended their studies to comparative physiology and ecophysiology. Most recently, evolutionary physiology has become a distinct subdiscipline.
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