394 HUMAN PHYSIOLOGY 



in the human is ordinarily given as 280 days. This is equiv- 

 alent to 10 menstrual cycles. Children born at the end of 

 8 or 9 of these cycles may survive, and it is of interest to 

 note that birth is more likely to occur at these times than 

 at intermediate dates. During pregnancy the usual signs 

 of menstruation are not in evidence, but there is still a 

 latent rhythm in the system. 



At the end of the full term the unborn child (fetus) has 

 reached an average weight of about 7 pounds. The uterus 

 has enormously enlarged to make room for the child, and 

 is, in fact, even larger than might be supposed, because the 

 fetus is surrounded by a considerable quantity of fluid 

 together with enclosing membranes and the placental 

 disc. The fluid leaves all parts of the growing body free 

 to develop without encountering hindrances by reason of 

 pressure. The uterus has, early in pregnancy, escaped 

 from the rigid confinement of the pelvis and risen high into 

 the abdomen. Its opening is obstructed. The nutritional 

 requirements of the mother during these months will be 

 seen to be high, and especially as regards the materials 

 necessary for the promotion of growth. It is not the child 

 alone which has to be considered, but the temporary in- 

 crease in the mass of the uterus, the accessory structures, 

 and also the mammary glands which are preparing to se- 

 crete milk. The harmonious advance of these associated 

 processes is probably secured by the action of internal 

 secretions proceeding from the cells of the ovary and from 

 the embryo itself. 



Birth. The separation of the child from the body of the 

 mother means that it is removed from a sphere .of auto- 

 matic care and protection to one in which these duties are 

 consciously and intelligently performed. It is at the same 

 time the beginning of responsible living for the baby. 

 Theoretically the process is a simple one. It is effected by 

 contractions of the smooth muscle of the wall of the uterus, 

 reinforced in the later stages by strenuous employment of 

 the abdominal muscles and the diaphragm. The first re- 

 sult of these activities is to dilate the neck (cervix) of the 



