318 HUMAN PHYSIOLOGY 



auricle into the right ventricle and pulmonary artery and 

 thence directly through the ductus Botalli into the aorta. 

 Only a small part of the blood pases through the lungs of 

 the embryo. This peculiar arrangement of the blood vessels 

 becomes clear when we recollect that the exchange of gases 

 in the embryo does not take place in the lungs and that 

 consequently only so much blood needs to flow through the 

 lungs as is sufficient to provide for their nourishment and 

 growth. After birth, when pulmonary respiration begins, 

 the division between the auricles of the heart is completed 

 and the ductus Botalli is obliterated. 



The placenta is a very vascular structure, composed of 

 two united parts, one part the maternal, the other the fcetal, 

 portion. The vascular villi of the foetal portion extend into 

 spacious blood cavities formed by the dilated capillaries of 

 the maternal portion. This great abundance of vessels in 

 the placenta, part of which belong to the foetus and part to- 

 the mother, makes a rapid exchange of gases between the 

 maternal and the fcetal blood possible. 



The formation of red blood corpuscles during the placenta! 

 circulation takes place chiefly in the liver and spleen of the 

 embryo. 



During the middle of pregnancy the cardiac sounds of the 

 embryo can be heard at different parts of the uterus accord- 

 ing to the position of the embryo. The double sound is 

 often accompanied by noises caused by the circulation of 

 blood in the umbilical cord. The rate of the cardiac sounds 

 of the embryo is 120-160 in a minute. It is increased by 

 movements of the embryo. 



(&) Respiration. In regard to the respiration of the em- 

 bryo, two periods can be distinguished. During the first 

 period corresponding to the vitelline circulation the supply- 

 ing of oxygen and removal of carbon dioxide is not brought 

 about by any special organs. Real respiration begins with 

 placental circulation. The taking up of oxygen and giving 

 off of carbon dioxide does not take place in the lungs but in 

 the placenta. The oxygen is supplied by the arterial blood 



