CHAP, in.] THE NUTRITION OF THE EMBRYO. 675 



connection with the maternal uterine sinuses, until at last in the 

 fully formed placenta the former are freely bathed in the blood 

 streaming through the latter, the nutrition of the embryo becomes 

 more and more confined to this special channel. The blood of the 

 foetus flowing along the umbilical arteries effects exchanges with 

 the venous blood of the mother, and leaves the placenta by the 

 umbilical vein richer in oxygen and nutritive material and poorer 

 in carbonic acid and excretory products than when it issued from 

 the foetus. 



As far as the gain of oxygen and the loss of carbonic acid are 

 concerned these are the results of simple diffusion. Venous blood, 

 as we have already seen, always contains a quantity of oxyhaemo- 

 globin, and the quantity of this substance present in the blood of 

 the uterine veins is sufficient to supply all the oxygen that the em- 

 bryo needs ; the blood of the foetus, containing less oxygen than 

 even the venous blood of the mother, will take up a certain though 

 small quantity. The foetal blood travelling in the umbilical artery 

 must, in proportion to the extent of the nutritive changes going on 

 in the embryo, possess a higher carbonic tension than that in the 

 umbilical vein or uterine sinus ; and by diffusion gets rid of this 

 surplus during its stay in the placenta. The blood in the umbilical 

 arteries and veins is therefore, relatively speaking, venous and arte- 

 rial respectively, though the small excess of oxyhsemoglobin in the 

 blood of the umbilical vein is insufficient to give it a distinctly arterial 

 colour, or to distinguish it as sharply from the more venous blood 

 of the umbilical artery, as is ordinary arterial from ordinary venous 

 blood. Thus the foetus breathes by means of the maternal blood, 

 in the same way that a fish breathes by means of the water in which 

 it dwells. 



The blood of the foetus is very poor in haemoglobin correspon- 

 ding to its low oxygen consumption. When the mother is asphyxi- 

 ated, the foetus is asphyxiated too, the oxygen of the latter passing 

 back again into the blood of the former ; and the asphyxia thus pro- 

 duced in the foetus is much more rapid than that which results 

 when the oxygen is used up by the tissues of the foetus alone, 

 as when the umbilicus is ligatured and the foetus not allowed to 

 breathe. 



If oxygen and carbonic acid thus pass by diffusion to and from 

 the mother and the foetus, one might fairly expect that diffusible 

 salts, proteids, and carbohydrates would be conveyed to the latter, 

 and diffusible excretions carried away to the former, in the same 

 way; and if fats can pass directly into the portal blood during 

 ordinary digestion, there can be no reason for doubting that this 

 class of food-stuffs also would find its way to the foetus through the 

 placental structures. We do know from experiment that diffusible 

 substances will pass both from the mother to the foetus, and from 

 the foetus to the mother ; but we have no definite knowledge as to 

 the exact form and manner in which, during normal intra-uterine 



432 



