476 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



an abundance of food containing all the chief kinds of food -stuffs. With the 

 alimentary canal in its embryonic and functionless state, this food, when it 

 reaches the embryo, must necessarily he already digested and ready for absorp- 

 tion by the cells. A supply of oxygen, not necessarily great in quantity, is 

 also needed. The fetal lungs are not ready for respiration, and the oxygen 

 must come to the blood by another channel than them. Carbonic acid must 

 be got rid of, and through other than pulmonary paths. Urea and its fore- 

 runners and other wastes, probably not in great quantity, must be excreted. 

 The fetal kidneys and the skin are probably never very active, as is made rea- 

 sonably certain by the late external opening of the male urethra, the late 

 development of the cutaneous glands, and the composition of the amniotic 

 liquid, into which they would naturally pour their secretions. Thus the paths 

 of income and outgo that are normal to the individual after birth are only 

 partially open during fetal life; nevertheless, the processes of income and 

 outgo must be performed. The placenta, with its close relationship but non- 

 communication of maternal and fetal blood-vessels, has, therefore, been evolved 

 phylogenetically, and appears early in the course of ontogeny. There is 

 brought to it on the part of the embryo and discharged into the villous 

 capillaries a mixed blood, comprising venous blood from the various capil- 

 lary systems of the body, and containing, therefore, the carbonic acid and 

 other wastes of venous blood, and a certain proportion of purified blood 

 which has passed directly by way of the (Indus venoms, the inferior vena 

 cava, the right auricle, the foramen ovale, and the left side of the heart to the 

 aorta and the umbilical arteries. There is brought to the placenta on the 

 part of the mother and discharged into the sinuses pure arterial blood, 

 laden with food and oxygen. Through the membrane intervening between 

 maternal and fetal vessels there pass from the fetus carbonic acid and other 

 wastes, and from the mother food (sugar, fats, proteids, etc.) and oxy- 

 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 role. 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 leucocyte- into the ictus may take place. 



