REPRODUCTION. 915 



gen. Back to the fetal liver and heart goes the nutritive and arterialized 

 blood, and back to the maternal excretory organs the vessels convey the fetal 

 wastes. The placenta is thus a peculiar organ intermediate between the living 

 cells of the embryo on the one hand and the digestive organs, lungs, kidneys, 

 and skin, of the mother on the other. Little is known of the actual details 

 of the placental process. The structure of the intervening cells indicates that 

 the interchange may be after a manner analogous to that taking place in the 

 lungs, rather than to that of a typical secreting gland i. e. that known physi- 

 cal processes, such as diffusion and filtration, play a prominent rdle. It has 

 been shown by several investigators that the fetus may be poisoned by car- 

 bonic oxide and strychnine, and may receive other harmless diffusible sub- 

 stances that are introduced in solution into the maternal circulation. The 

 mother may be affected similarly from the fetal circulation. But, as in the 

 case of the lungs, so the placental membrane can scarcely be regarded as 

 acting in the same passive way as a lifeless membrane would act (compare 

 Respiration). As accessory to the main nutritive source it has been sug- 

 gested that a diapedesis of maternal leucocytes into the fetus may take place. 

 The uterine glands are thought by some to afford a nutritive secretion to the 

 sinuses, and to the amniotic fluid has been ascribed a nutritive function. 

 Theoretically, these various means are not impossible, but true placental diffu- 

 sion must be regarded as the chief principle at work. The result is that the 

 mother relieves the child of all the labor of nutrition except that connected 

 directly with the latter's own cellular and protoplasmic metabolism. The 

 fetal energies are, therefore, free to be expended in the process of growth, 

 while gestation profoundly affects the maternal organism. 



Physiological Effects of Pregnancy upon the Mother. As might have 

 been expected, there is probably not one organic system within the mother's 

 body that is not more or less altered by pregnancy, often morphologically, but 

 especially in regard to function. And such normal alterations pass so gradu- 

 ally and so frequently into genuine pathological conditions that it is sometimes 

 difficult to draw the line between the two. The most marked changes are 

 connected with the body of the uterus, and have already been described. The 

 walls of the cei'vix uteri become hypertrophied, though to a less degree than 

 the body, and their glands secrete a quantity of mucus that forms a plug com- 

 pletely closing the passage-way of the cervix (Fig. 314). The rest of the 

 reproductive organs from the uterus outward become involved in the increased 

 venous hyperaBmia. The walls of the vagina become infiltrated with serous 

 fluid. The parts of the vulva partake in the general tumefaction. From the 

 second month of gestation onward the mammary glands undergo gradual devel- 

 opment as a preparation for the post-partum lactation. The increase in size of 

 the laden uterus brings gradually increasing pressure to bear upon the abdom- 

 inal viscera, and thus mechanically causes functional derangements of the 

 digestive and the urinary organs. The stretching of the abdominal skin 

 results in localized ruptures of the connective tissue of the cutis, the charac- 

 teristic scars forming the since gravidarum, which persist after pregnancy. 



