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ESSENTIALS OF PHYSIOLOGY. 



The cell thus formed at once divides, and when the ovum which has 

 been fertilised in the Fallopian tube reaches the uterus, it has already 

 divided and subdivided to form a small mass of cells called a inorula. 



The further development of the morula takes place in the uterus 

 and constitutes pregnancy. The morula makes its way into the uterine 

 mucous membrane, which then consists of three parts : namely, (1) the 

 decidua basalis, lying between the embryo and the muscular wall of the 

 uterus ; (2) the decidua capsularis (reflexa), between the embryo and the 

 cavity of the uterus; and (3) the decidua vera, lining the remainder of 

 the uterus. As the embryo grows, it becomes enclosed in a sac filled 

 with fluid and called the amiiion ; surrounding the amnion is a vascular 

 membrane, the chorion, from which blood-vessels pass to and from the 

 foetus in the umbilical cord. After the third month of pregnancy the 

 foetus receives its nutrition from the placenta, which is formed partly 

 from maternal and partly from foetal tissue. It consists essentially of 

 large blood spaces in the decidua basalis, into which open the uterine 

 arteries, and from which the blood of the mother is carried into the 

 uterine veins. Projections from the chorion (chorionic villi) containing 

 foetal blood-vessels lie in these spaces and are bathed by the maternal 

 blood \ the blood reaches the villi along the umbilical arteries, and is 

 returned to the foetus in the umbilical vein. The maternal and foetal 

 blood are thus separated by a double layer of epithelium, and the 

 nutrition of the foetus is effected by the diffusion of oxygen and 

 nutritive material through this epithelium. ' 



Parturition. The average duration of pregnancy is 280 days, 

 during which the muscular wall of the uterus not only increases in 

 size, but becomes greatly thickened. Parturition is brought about by 

 rhythmic contraction of the uterine muscle, and the foetal membranes 

 and their contained fluid are forced through the os uteri, which becomes 

 fully distended. This, the first stage of labour, ends when the os uteri 

 is fully dilated ; and the membranes rupture about this time. 



The foetal head then enters the pelvis, and the uterine contractions 

 become more prolonged and frequent, being accompanied by voluntary 

 contractions of the abdominal muscles. The foetus is gradually forced 

 through the pelvic canal and vulva, the head normally being born first. 

 The second stage of labour ends when the child is born. The whole 

 process of parturition varies greatly in duration, and may last twenty- 

 four hours. Shortly after the birth of the child the uterus contracts 

 further and expels the placenta. 



After parturition the uterus rapidly decreases in size, this being 

 known as involution. 



The Mammary Gland. The mammary gland consists of a number 



