REPRODUCTION 1013 



Exchange of Materials in the Placenta. The maternal blood, as 

 it streams through the colossal capillaries of the decidua, gives up 

 to the foetal blood oxygen and food substances and receives from it 

 carbon dioxide and in all probability urea. It is true that the blood 

 in the uterine sinuses is not itself fully oxygenated ; it is not bright 

 red arterial blood. But it yet contains more oxygen, and oxygen at 

 a higher partial pressure (p. 248), than the purest blood of the foetus, 

 and is, therefore, able to part with some of the surplus to the dark 

 stream of oxygen -impoverished blood brought by the umbilical 

 arteries to the placenta. Thus, it has been found that while the 

 blood of the umbilical artery of the fetus of a sheep had 47 volumes 

 per cent, of carbon dioxide, and only 2 '3 of oxygen, that of the 

 umbilical veins had 6*3 volumes of oxygen, and only 40^5 of carbon 

 dioxide (Zunt? and Cohnstein). In the exchange of gases between 

 the placental and the foetal blood the same general features present 

 themselves as in the external and internal respiration of the mother, 

 with this difference, that the exchange of oxygen is neither between 

 air and haemoglobin, as in the lungs, nor between haemoglobin and 

 tissue elements, as in the organs; but between maternal and foetal 

 haemoglobin, of course, through the mediation of the maternal and 

 foetal plasma. There is no reason to suppose that the mechanism 

 of the exchange is essentially different from that of the more familiar 

 forms of respiration. Diffusion of the gases from places of higher 

 to places of lower tension unquestionably plays an important 

 part. But this does not exclude the possibility of a more active 

 process of some other kind, although there is at present no direct 

 evidence of such a gaseous secretion as has been previously discussed 

 in connection with pulmonary respiration (p. 258). The presence of 

 oxydases in the placenta does not throw any light on the question. 

 For there is no proof that they act in transferring oxygen from the 

 one circulation to the other, and oxydases are found in the most 

 diverse tissues. Their significance for the combustion processes of 

 the body has already been alluded to (p. 264). 



Salts soluble in water, including not only those necessary for 

 nutrition, like sodium chloride, but many foreign salts, pass readily 

 from the placenta to the foetus, and in general more easily the lower 

 their molecular weight. Such salts as potassium iodide, e.g., when 

 injected into the maternal circulation, appear in the foetus in a very 

 short time. On the other hand, colloidal solutions e.g., of silver 

 or silicic acid do not pass over at all. It is of practical importance 

 that substances like chloroform, ether, and other narcotics, and alka- 

 loids like morphine and scopolamine, when administered in obstetrica 

 practice, may find their way from the mother to the child, although 

 more slowly and more capriciously than the salts. While diffusion 

 and osmosis assuredly take part in the passage of materials from the 

 placenta to the foetus, there is no more reason to conclude that the 

 whole exchange, even for the salts, depends upon such simple physical 

 processes than there is in the case of the exchange between any one 

 of the maternal tissues and the maternal blood. The essential 

 similarity of placental and intestinal absorption, to take one instance, 

 is seen in the mechanism by which the foetus gains the iron required 

 for the development of its haemoglobin. The haemoglobin of the 

 mother appears to be the most important source of this iron. 

 Erythrocytes in all stages of decomposition can be found in con- 

 tact with the chorionic villi, and even in the epithelium covering 

 the villi. These corpuscles come partly from extravasations in 



