CHAP. IT.] PREGNANCY AND BIRTH. 385 



activity of a secretory kind. The placental respiration of the 

 mammal seems in fact exactly to repeat the branchial respiration 

 of the fish ; in the former the foetus breathes by means of the 

 maternal blood in the same way that in the latter the fish breathes 

 by means of the water in which it lives. 



It follows from the above that the foetus may be asphyxiated 

 in two ways : on the one hand by interference with the access of 

 foetal blood to the placenta, as when the cord is tied, and on the 

 other hand by the maternal circulation being arrested, or by the 

 maternal blood being wanting in oxygen. When the mother is 

 asphyxiated the foetus is asphyxiated too, the oxygen passing 

 from the foetal blood to that of the mother. In such a case, owing 

 to the more imperious demands of the maternal blood, the store of 

 oxygen in the foetal blood is sooner exhausted and asphyxia is 

 more rapidly developed than in the case when the cause lies 

 in the foetus, not in the mother, and the oxygen simply dis- 

 appears from the foetal blood as it is slowly used up by the 

 foetal tissues ; for the rate of foetal oxidation though it increases 

 continually during the intra-uterine life, especially in the later 

 stages, is slow compared to what it becomes some time after birth. 



957. The foetus not only breathes but also feeds and 

 probably excretes by means of the placenta ; the blood returning 

 by the umbilical vein is not only richer in oxygen and poorer in 

 carbonic acid but also richer in nutritive material and poorer in 

 waste products than the blood of the umbilical arteries. In deal- 

 ing however with the nutrition of the embryo we must bear in mind 

 a special condition under which the embryo lives. As we have said 

 the embryo proper becomes at an early date invested with the 

 double membranous bag of the amnion, consisting of the inner 

 amnion and outer (false) amnion. Between the two there is at 

 first a space, into which as we have seen the allantois grows in 

 order to become the placenta ; but, as the fluid, which from the 

 first is present within the inner bag, increases in amount, without 

 any corresponding increase in the fluid between the inner and 

 outer bag, the (true) amnion in its expansion after the formation 

 of the placenta reaches and unites with the false amnion which by 

 this time is known as the chorion. The whole interior of the 

 uterus is lined, next to the decidua. by a membrane apparently 

 simple but composed of united amnion and chorion, and within 

 this, surrounding and supporting the embryo, lies the amniotic 

 fluid, which at first scanty rapidly increases in amount until in the 

 later stages of pregnancy it may amount to 800 c. c. or even much 

 more. 



In the roof of the uterus, in the region of the placenta, the 

 amniotic fluid is in close proximity not only to the branching 

 umbilical arteries and veins of the foetus, but also to many of the 

 maternal blood vessels being separated from the maternal blood 

 by nothing more than the thin wall of the blood vessel and the 



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