CHAP. IL] PREGNANCY AND BIRTH. 1521 



sume very little oxygen compared with the processes leading to 

 expenditure of energy in movement and heat. Hence the simple 

 method of nutrition and respiration by means of the direct contact 

 of the cells of the uterine mucous membrane is exchanged for the 

 special vascular mechanism of the placenta, by which the embryo 

 lives upon and breathes through the uterine blood of the mother. 

 From an early period up to birth the placental circulation is the 

 chief, we may almost say the only means by which the embryo 

 breathes and is fed ; but the details of the placental events are 

 changing during the whole of this time. The embryo, all the while 

 increasing in bulk, passes through phase after phase ; the structural 

 features of one day give way to those of the next, its morpho- 

 logical history being as it were a series of dissolving views ; and 

 each new structural phase entails new functional events both in 

 the embryo itself and in the placenta. This is perhaps especially 

 seen in the earlier stages at a time when the placental circulation 

 has been established in its main outlines, but in the embryo most 

 of the future organs are still in a shadowy inchoate condition. 

 At this epoch, of the total bulk of blood coursing from the embryo 

 towards the tissues of the mother and back again, the greater part 

 is at any one moment to be found in the placenta and only a small 

 part in the tissues of the embryo itself; later on the blood is equally 

 divided between the placenta and the embryo ; and still later the 

 embryo has the larger share, and it is the smaller part which is at 

 any one moment flowing through the chorionic villi of the placenta. 

 There can be no doubt that in the earlier phase the influences which 

 the placental structures exert on the foetal blood are in many ways 

 different from those which are exerted later on. We find that 

 during the earlier phases the cellular placental elements are 

 correspondingly prominent, indicating that much labour of the 

 kind for which cells are necessary is being then carried on, 

 whereas in the later stages the placental mechanism approaches 

 though it never quite reaches the more mechanical condition of 

 a simple membrane separating the foetal and maternal blood. We 

 cannot enter at all fully here into the successive phases ; we must 

 confine ourselves chiefly to the main features of what is going on 

 during the latter months of gestation when the placental circula- 

 tion is in full swing. 



956. At this time the somewhat rapid strokes of the foetal 

 heart drive the foetal blood through the umbilical arteries to 

 the capillaries of the chorionic villi, from whence it is returned 

 by the umbilical vein. From experiments on lambs and other 

 animals it would appear that the blood pressure in the um- 

 bilical artery is moderately high (40 to 80 mm. Hg.) and that in 

 the umbilical vein very considerable (15 to 30 mm. Hg.), higher 

 than the venous pressure in the mother in a vein of corresponding 

 size ; the difference between the arterial and venous pressure is 

 therefore relatively less than in the mother. Corresponding to this 



