DK Eyewitness Human Body

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A new life; Steve Parker * c) T4 |" _! y4 U, y0 c
DK Eyewitness Human Body   11-01-2004 * }4 s/ U& e4 {: x$ @
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The time in the womb is the first, yet largely unseen, part of a baby's life. It is the period between an egg joining with a sperm cell at fertilization, and the resulting baby's entrance into the world, at birth. A female human is pregnant for about 266 days. A female mouse is pregnant for about 20 days, and a female elephant for some 660 days. Our species follows the general trend in the mammal group – the larger the body size, the longer the pregnancy. In fact, almost everything about pregnancy in humans follows the standard mammal pattern. Knowledge of pregnancy has come a long way since Aristotle's views. He stated that the father provided the soul and spirit for the baby. The mother provided the menstrual blood, which mixed with semen fluid from the father, and remained in the womb to give rise to the baby. The mother was thought to be simply a carrier or "incubator," feeding and providing for the life force that originated from the man.* I! c/ X6 I+ D! I3 c/ P

8 V" j2 `& }( h& Q" }s-e-xual intercourse
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In the 1950s and 1960s, scientists carried out detailed research into s-e-xual intercourse. The woman's vagina expands and moistens with mucous secretions. The man's penis fills with blood, and becomes erect. In the orgasmic phase of the male, the muscles around the s-e-x organs contract rhythmically, and sperm cells in their semen fluid are ejaculated into the woman's vagina. After orgasm, the muscles and s-e-x organs relax and return to normal, and the partners sometimes feel drowsy.
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First embryologist ' W" `2 b( i1 ?6 f

$ D  v2 \$ R6 t( ~' WIn 1604, Italian professor Hieronymus Fabricius (1537-1619) published De Formato Foetu, a study about the development of eggs and unborn babies in a range of animals, including humans. Even in his lifetime, Fabricius was known as the founder of embryology. He named the ovary, the egg-mak-ing organ in the chicken, and predicted its function.
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Embryo development
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! S. P+ [. P# p& [5 KThe fertilized egg divides into two cells then into four, then eight, and so on, every few hours. The resulting hundreds, and then thousands of cells gradually group together, and change into different types such as muscle, nerve and blood cells. Five weeks after fertilization, the embryo is smaller than a baked bean, yet its arms and legs are already developing.
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Fetal development
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About two months after fertilization, all of the major organs have formed, and the baby's heart is beating, and yet its body is still just 1 in (25 mm) long. Embryologists through the ages have been amazed at the similarity between the early embryos of humans, monkeys, cats, and many other types of mammals. From two months through to birth the developing baby gradually comes to look distinctly human. It is known as a fetus, from the Latin for "offspring."
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Twin babies   a+ v# Z3 u, l9 q0 |/ T

# N$ p' l/ i/ ]3 }Roughly one pregnancy in 90 results in two babies. Two-thirds of these are non-identical twins, who develop from separate eggs if the woman released two egg cells on ovulation, and each egg was fertilized by its own sperm. Each twin has its own set of membranes that protect it, as in this model, and it also has its own placenta for nourishment. Identical twins develop from the same fertilized egg. They are always the same s-e-x and share the same placenta and amniotic membrane, and the same genes.

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Birth and baby; Steve Parker " z1 e6 p7 w/ I/ k
DK Eyewitness Human Body   11-01-2004   ]2 y3 |: b3 b7 Y2 A4 z' D
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1 g7 L; f* z; z$ P0 UIn theory, the process of childbirth is relatively sim-ple. The neck, or cervix, of the mother's uterus, which is tightly closed during pregnancy, relaxes and widens (dilates). The uterus muscles begin to tense and shorten in waves called contractions, which become more frequent and powerful. This stage is called labor – aptly named, since it is hard work for mother and baby. Gradually the contractions push the baby through the dilated cervix and along the vagina to the outside world. This is stage two, delivery. It is followed by stage three, afterbirth, when the placenta emerges. In practice, childbirth experiences vary. Sometimes the labor is slow, or the baby becomes stuck. Methods used to "help" the birth have ranged from the ineffectual to the barbaric. The surgical Caesarean delivery is named after Roman Emperor Julius Caesar (100-44 BC), supposedly born this way. In the past it was usually carried out if the mother died. Only this century have mother and baby had a reasonable chance. The medical speciality of childbirth is obstetrics, from the Latin obstare. This means to "stand opposite," referring to the midwife who stood near the mother, and helped her through the event.% B8 y. {; b) h& [9 Z1 D
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Mother and her newborn child 8 Z0 [- r0 _+ k3 s2 |$ ^4 `/ G
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The minutes and hours soon after birth are very important for mother and new baby. They quickly learn to recognize each other, particularly by scent, a sense which is deeply rooted in the emotional parts of the brain. The human mother-baby bond is extremely powerful, as it is even in wild animals.
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Ready to leave ' [) S2 Z5 `+ [% q. q( Z

8 `7 |% K" L& j. B0 u3 Y) eNine months after fertilization, the full-term baby is ready to leave the warm, watery darkness of the uterus. Most babies rest head down so that they emerge through the birth canal with the widest and roundest part first – the head. The rest of the body then follows easily and slips out. Other positions, or "presentations," such as buttocks-first (breech) or shoulder first, are more likely to have complications.
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: J6 A1 U: O* |) H& f% e* KThe birth canal
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* J& F& i5 U: v: |  H$ O/ jThe baby passes through the large central hole in the mother's pelvis, or hip bones. This is much broader than the hole in a man's pelvis, to accommodate the shape of the baby's head. The pelvic joints and ligaments, particularly the pubic symphysis between the two pubis bones, stretch slightly to make the hole larger.9 X: q  w) |7 [1 k/ A- t
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Surgical delivery   ^6 k; z  u# r' O
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In a Caesarean delivery, or Caesarean section, the baby emerges through an incision, or cut, made through the mother's skin, abdomen, and uterine muscles. These days this O p e r ation is very safe, and leaves only a small scar on the mother's lower abdomen. Caesareans are performed for a number of reasons. The baby may be stuck in the birth canal and growing distressed, or at risk from lack of oxygen.
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The afterbirth
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8 _, s& W' H& j/ r+ v$ q% N5 {After the baby is born, the placenta comes away from the lining of the uterus, and it is delivered in the same way, which is why it is usually known as the "afterbirth." The baby is linked to the placenta by the umbilical cord, along which blood flows in between baby and placenta. During pregnancy, the placenta brings together the baby's blood and the mother's blood, so that they are separated only by an extremely thin membrane. Oxygen and nutrients to help the baby grow pass from mother's blood to baby's blood, while the baby's wastes, like urea and carbon dioxide, move in the opposite direction for disposal.
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* H0 _6 ~" h. H% w! }The word "mammal" comes from the presence of mammary glands in the female of the species. These make milk, or lactate, to nourish the baby. The human mammary glands are in the breasts on the chest. Each contains about 20 lobes, which are clusters of milk-mak-ing lactiferous glands. During pregnancy and after birth the glands enlarge and produce milk, which flows along lactiferous (milk) ducts, and oozes from the nipple. When the baby sucks at the nipple, this stimulates milk to flow from the breasts.