CHAP, ii.] PKEGNANCY AND BIRTH. 11L'5 



ing size ; the difference between the arterial and venous 

 pressure is therefore relatively less than in the mother. Cor- 

 responding to this 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 46 p.c. of carbonic acid. Hence the blood in the 

 umbilical vein is essentially arterial blood, and that in the 

 umbilical arteries essentially 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 f ratal arterial blood is far from being 

 saturated with oxygen, whereas as we have seen ( 286) 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 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 ex- 

 posed 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 umbilical 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 



