384 THE NUTRITION OF THE EMBRYO. [BOOK TV. 



the velocity of the blood flow is relatively low. The number of 

 red corpuscles in a given bulk of foetal blood, which was of course 

 at first very scanty, has by this time much increased, but as a rule 

 remains up to the end less than that of the mother, though this 

 has become diminished by the pregnancy. In many cases no 

 marked distinction of colour can be observed between the blood in 

 the umbilical arteries and that in the umbilical vein, but such 

 difference as can be noted is in the direction of the blood in the 

 vein being brighter than that in the arteries, and at times this is 

 conspicuously the case. If, for instance, the foetus at the time 

 of observation happens to make prolonged movements, the contrast 

 between the dark blood of the umbilical arteries and the bright 

 blood of the umbilical vein may become striking. An examination 

 of the gases of the blood shews that the blood in the vein contains 

 more oxygen and less carbonic acid than that of the arteries ; the 

 former for instance has been found to contain from 7 to 20 p.c. of 

 oxygen and 40 p.c. of carbonic acid, the latter 2 to 6 p.c. of oxygen 

 and 40 p.c. of carbonic acid. Hence the blood in the umbilical vein 

 is essentially arterial blood, and that in the umbilical arteries essen- 

 tially venous blood. It may be observed that while as regards the 

 amount of carbonic acid the blood of the foetus runs parallel to that 

 of the mother, the arterial blood of the foetus (in the umbilical vein) 

 contains less oxygen than that of the mother. This is not due 

 alone to the relatively smaller amount of haemoglobin, for as shewn 

 by experiment the haemoglobin of the foetal arterial blood is far from 

 being saturated with oxygen, whereas as we have seen ( 355) that 

 of the adult is, or very nearly so. We may add that the foetal 

 blood left to itself uses up its free oxygen rapidly, very much more 

 rapidly than does adult blood. 



The maternal blood is conveyed, as we have seen, to the 

 placental sinuses by arteries which open directly into the sinuses. 

 Hence, though independently of any influence exerted by the 

 foetal blood the blood returned from the sinuses by the uterine 

 veins is venous blood, rendered venous by the maternal tissues 

 themselves, yet the blood in the sinus to which the capillaries 

 of the villi are exposed may be regarded as rather arterial 

 than venous, and in any case contains more oxygen and less 

 carbonic acid than does the foetal blood arriving by the um- 

 bilical arteries. Seeing that the relatively narrow uterine 

 arteries open out suddenly in the wide placental sinuses the 

 flow in the latter must be slow ; the flow in the foetal vessels is also 

 as we have seen not rapid; hence ample time is given for the 

 interchange of gases. The change which is thus effected is probably 

 carried out by diffusion, the amount of change being determined 

 by the relative percentages of the gases in the maternal and foetal 

 blood. At least we have no more evidence in the case of this 

 placental respiration than we had in the case of the pulmonary 

 respiration that the interchange is in any way assisted by cellular 



