384 THE NEW INDIVIDUAL Part IV 



Other hormones produced by the placenta include estrogen and progesterone 

 which stimulate the enlargement of the uterus, the growth of the mammary 

 glands, and are involved in the uterine contractions that occur at birth. 



Birth. The birth process begins with rhythmic contractions of the smooth 

 muscles in the uterine wall, joined later by the striated muscles of the ab- 

 dominal wall. These are timed with the stretching of the birth canal so that 

 the infant is forced out, normally head first, pulling the umbilical cord after it. 

 Similar contractions expel the afterbirth, which includes the placenta and 

 all the other membranes which were, for a time, of life and death importance 

 to the infant. Birth is in no way such a simple process as this statement sug- 

 gests. A complex of hormones, changed rate of blood flow, sensitivity of nerves 

 and muscles — a whole system of balanced forces — is concerned. 



At birth, a baby meets a great crisis of its life. For nine months it has 

 lived in a soft-walled chamber, flooded with fluid warmed to a steady 98.6° 

 F., protected from jar and vibration and in total darkness (Fig. 19.18). 

 Food ready to use and oxygen have been filtered into its blood. Its lungs 

 are collapsed, without air and with only a fraction of the blood soon to 

 come to them. Instead of going to the lungs, the main supply of blood has 

 taken a short cut and bypassed them; it also has crossed the heart through 

 an opening between the auricles. These arrangements provide for the circu- 

 lation to the placenta; after birth, they would be useless and worse. If the 

 short routes stay open, a blue baby results because venous blood leaps through 

 the opening from the right to the left auricle, and through the duct from the 

 pulmonary artery into the aorta (Fig. 19.19). 



When a baby first emerges into the air its lungs are immediately inflated 

 due to the negative pressure in its lungs and the positive pressure of the air. 

 It must breathe, at once and without practice, a complicated business in 

 which failure is fatal. Before birth, the baby may only swallow amniotic 

 fluid and whatever it contains. After its birth, it deals with food at first hand; 

 its digestive tract is new to this also, the reason for hiccoughs and other 

 digestive rebellions. A baby arrives in a changeful environment, of moving 

 air that may be dry or moist, of shifting temperatures, changing light, food 

 in variety, human neighbors, plants and animals. With unwarned suddenness 

 its ecology is changed and it begins adjustments that must continue through- 

 out its life. 



Twins. Multiple births are due to the development of more than one egg 

 or to the division of the fertilized egg into parts each of which develops into 

 an infant. 



Fraternal twins are the product of two different eggs which matured at the 

 same time and were fertilized by two different sperm cells. Fraternal twins 

 have different genes and are not any more alike than any children of the 

 same parents. They may or may not be of the same sex. There is a separate 



