CHANGES DURING PREGNANCY 515 



absence of pain and the consequent loss of the reflex contractions of 

 the abdominal muscles. The most important nervous elements for 

 the uterus are contained in the uterine, paracervical, and paravaginal 

 ganglia, but their excitability to external stimuli gradually decreases 

 during pregnancy and is lost at the end. 



We are thus forced to conclude that the phenomena of pregnancy 

 and parturition are brought about by chemical stimuli acting through 

 the blood-stream. The hormone or hormones may arise in the corpus 

 luteum, which is essential for the progress of pregnancy, at least in 

 the early stages (Marshall and Jolly 1 ). Evidence is also forthcoming 

 that the mammary secretion is due to an ovarian influence in certain 

 cases. For instance, secretion may occur in virgin women who are 

 the subjects of ovarian tumours, and in virgin bitches. Cramer 2 has 

 recorded a case in which the transplantation of ovaries into a woman, 

 whose genital organs were much atrophied, led to a secretion of 

 colostrum. 3 On the other hand, removal of the ovaries at the middle 

 of pregnancy does not interfere with the second half of the period of 

 gestation, or with labour and lactation. 



The presence of the placenta may modify the normal metabolism 

 in various ways. It is set in the path traversed by the formative 

 material on its way to the embryo, and by the waste products excreted 

 by the embryo. The form in which the materials required by the 

 product of conception reach the placenta is still obscure. The protein 

 may be merely the " circulating protein " found in the non-pregnant 

 condition, or more highly elaborated. The diffusion of the blood-sugar 

 to the foetus is disputable, 4 and the form of the fats is unknown. Of 

 the waste products carbonic acid, which, according to Bohr, 5 results 

 entirely from the combustion of carbohydrates in the mammalian 

 foetus, is excreted into the maternal circulation through the placenta. 

 With regard to a wastage in the protein metabolism, a certain loss 

 is bound to occur in the transformation of "circulating" or "fixed" 

 maternal proteins into foetal tissue proteins ; and in addition, incom- 

 pletely oxidised substances may possibly be transmitted to the 

 placenta and oxidised there or in the mother (Bohr 6 ). The question 

 of urea formation by the fretal liver or the trophoblast still awaits 

 investigation. The only observation bearing on this point is that 



1 Marshall and Jolly, " The Ovary as an Organ of Internal Secretion," Phil. 

 Trans. Roy. Soc., London, B., vol. cxcviii., 1905. 



2 Cramer (H.), "Transplantation menschlicher Ovarien," Munch, med. 

 Woch., 1906. 



3 With regard to the existence of an ovarian stimulus, see also Hildebrandt 

 (ffofmeisterfg Beitrdge, vol. v., 1904). s 



' 4 See Chapter X., p. 462. 



'"' Bohr, " Dei 1 respiratorische Stoffwechsel des Saugetierembryo," Skand. 

 Arc/i. d. F/ii/g., vol. x., 1900 ; also vol. xv., 1904. 



6 Bohr, see Nagel's HatncRntck der Physiologw, " Respiration," vol. L, H. i. 



