RESPIRATION IN THE EMBRYO AND FCETUS 571 



placenta acted as a lung was not fully recognised until Zweifel 

 in 1876 showed that the spectrum of oxy-haemoglobin could be 

 clearly seen in the umbilical cord before the child breathed by 

 its lungs. Zweifel also demonstrated that the blood in the 

 umbilical vein of a foetal rabbit was brighter than that in the 

 arteries, if the precaution were taken to open the uterus of the 

 mother in warm normal saline solution and thus prevent vigorous 

 contractions of the uterus : the difference in the colour of the 

 vessels disappeared during asphyxia but reappeared when artificial 

 respiration was performed upon the mother. 



The mechanism of the fcetal respiration was more fully ex- 

 plained by the experiments of Zuntz, who showed that during 

 asphyxia of the mother the blood of the umbilical vein became 

 darker than that of the umbilical arteries, the foetal blood yielding 

 up oxygen to the blood of the dying mother ; the umbilical vein 

 became as dark as the arteries when the maternal blood-vessels 

 supplying the placenta were compressed ; the blood of a foetus 

 respiring air by its lungs lost oxygen in the placenta, which was 

 attached to an excised piece of the uterus ; the umbilical vein 

 coming from the intact placenta contained blood as bright in 

 colour as the arterial blood of the uterus during the normal 

 respiration of the mother, and foetal movements made the blood of 

 the umbilical arteries darker in colour. 



Zuntz in conjunction with Cohnstein also analysed the blood 

 of the umbilical artery of a foetal sheep about three weeks before 

 full term ; the total gas was 54-211 volumes per cent., of which 

 6-669 were oxygen, 46-542 carbon dioxide, and 1 nitrogen. Com- 

 parative analysis showed that the blood in the umbilical artery 

 contained 4-67 volumes per cent, less oxygen and 4-72 more carbon 

 dioxide than the blood in the umbilical vein. These observers 

 calculated that the foetal respiratory exchange was relatively much 

 less than that of the adult ; but there are great difficulties to over- 

 come before exact data can be obtained for such an estimation, 

 which must be based upon comparative analysis and determina- 

 tion of the rate of flow of the blood through the placenta ; operative 

 procedures quickly disturb the circulation. From these experi- 

 ments it is concluded that the foetus respires by processes of diffusion 

 in the placenta ; under normal conditions the pressure of oxygen 

 in the maternal blood, which supplies the uterus, is higher than 



