REPRODUCTION 



545 



general circulation. This is necessary be- 

 cause the blood must be forced through the 

 paired umbilical arteries to the placenta, 

 which is a highly vascular organ requiring 

 large quantities of blood. The blood flows 

 in this path throughout fetal life, while the 

 organ systems are being developed so that 

 at the moment birth occurs they will func- 

 tion properly. 



The basic structure of the human embryo 

 is well delineated by the end of the twelfth 

 week of intrauterine life, although there is 

 a great deal of growth that must take place 

 before the child is ready for the rigorous 

 life that awaits him in the outside world. 

 The uterine wall keeps pace with the grow- 

 ing embryo and the added burden gradu- 

 ally changes the entire body contour of the 

 mother (Fig. 21-12). As the fetus grows it 

 slips posteriorly, with its head low in the 

 pelvis, and by the fortieth week of its life 

 it is ready to be born. 



Birth 



The process of being born is one of the 

 more remarkable events in the biological 

 world (Fig. 21-13). As the fetus passes 

 through tlie tight-fitting birth canal, the 

 umbilical cord remains attached to the pla- 

 centa, and until the moment the child takes 

 his first breath nourishment and oxygen are 

 received from this source. Upon exposure 

 to the external world the usual stimulation 

 coming from being chilled and from the 

 high CO2 in his blood causes the child to 

 take his first breath. The partial vacuum 

 created in the chest cavity causes the blood 

 to be diverted from the ductus arteriosis 

 into the pulmonary artery and lungs. This 

 is aided by a powerful sphincter muscle sur- 

 rounding the ductus arteriosis which con- 

 tracts at this moment and never relaxes. The 

 large volume of blood then makes its way 

 back to the heart through the pulmonary 

 veins, filling the left auricle. Since this 

 chamber is now filled from a new source, 

 it is no longer necessary for an opening to 

 exist between the auricles, hence a flap of 



12 

 weeks 



Fig. 21-12. Changes occur in both the uterus and the 

 mother to accommodate the developing fetus. 



