572 THE RESPIRATORY EXCHANGE 



that of the oxygen in the blood of the umbilical artery ; during 

 asphyxia the reverse is the case, and the mother's blood absorbs 

 oxygen from the foetal blood. The transference of carbon dioxide 

 is probably effected by diffusion. Further experiments, however, 

 are needed to show whether the placenta possesses any power 

 of secreting gases. 



Bohr ( 37 ) has estimated the respiratory exchange of the foetus 

 in another way ; he determined the absorption of oxygen and the 

 discharge of carbon dioxide in a pregnant guinea-pig before and 

 after the umbilical cords of the foetuses had been compressed. 

 His results show that weight for weight the respiratory exchange 

 is a little greater than that of the mother. The foetus, although 

 it is shielded from loss of heat and shows no marked muscular 

 activity, is rapidly growing and the chemical changes involve a 

 rapid respiratory exchange. 



In conclusion, what is the nature of the foetal apnoea ; is it pro- 

 duced by a low pressure of carbon dioxide and a high pressure 

 of oxygen, or are other factors present ? This question requires 

 further investigation before it can be answered. It is true that 

 the blood in the umbilical vein contains about 6-3 volumes per 

 cent, of oxygen and 40-5 of carbon dioxide ; but it is not this blood 

 but mixed blood which supplies the respiratory centre. The blood 

 in the umbilical artery contains about 2-3 volumes per cent, of 

 oxygen and 47 of carbon dioxide ; but the pressures of the gases, 

 not their quantities determine excitation, and they are unknown. 

 The respiratory centre responds to compression of the umbilical 

 cord, and that is practically all that is known beyond the fact that 

 its excitability appears to be lower and less easily destroyed than 

 that of the adult animal. 



BIBLIOGRAPHY 



The following list is not a complete one, for it is largely confined to those 

 works which can be readily consulted by the student. Numerous references 

 to the literature of the subject are given in the works marked with a star. 



GENERAL 



Foster, The History of Physiology, Cambridge (1901). 



*Zuntz, Physiologie der Blutgase und des Respiratorischen Gaswechsels. 

 Handbuch der Physiologie von Hermann (1882), Vierter Band, 2 ter Theil, 

 S. 1. 



