184 Prof. Draper on the Cause of the 



The Placental Circulation. — The umbilical arteries 

 carry in their spiral courses, as they twist round the umbilical 

 vein, the effete blood of the foetus, and distribute it by their 

 ramifications to the placenta. In that organ it is brought in 

 relation with the arterial blood of the mother, which oxidizes 

 it, becoming by that act deoxidized itself. The foetal blood 

 now returns along the ramifications of the umbilical vein, and 

 finally is discharged from the placenta by that single trunk. 



That this is truly a change similar to that which is accom- 

 plished in the adult lungs, is shown by the circumstance that 

 the blood of the umbilical arteries becomes brighter on its 

 passage into the umbilical vein. 



As the venous blood of the foetus is thus oxidized by the ar- 

 terial blood of the mother, movement must of necessity ensue 

 in it, on the same principle that it ensues in the adult lung, and 

 must take place in the same direction, that is to say, from the 

 venous to the arterial side. 



The fcetal circulation offers a very close resemblance to the 

 circulation of fishes, and is merely a refined variety of that 

 type. The true difference is that in fcetal life the condition 

 of immobility is observed. In fishes the venous blood is 

 brought to the gills, and subjected in their fibrillary tufts to 

 the oxidizing agency of the air dissolved in the surrounding 

 water. In these organs it therefore becomes arterialized, and 

 is pushed into the pulmonary veins. These empty directly 

 into the aorta, no systemic heart intervening, and the mecha- 

 nical impulse received by the blood during its oxidation is 

 found sufficient to carry on the aortic circulation : the heart 

 therefore may be and is dispensed with. A fish, by sponta- 

 neously changing its position, or by the mechanical establish- 

 ment of currents in the surrounding medium, can obtain new 

 surfaces of water for the oxidation of its blood ; but for the 

 motionless fcetal mammalian a higher mechanism is required, 

 a mechanism which can bring the oxidizing-maternal-arterial 

 blood in relation with the branchial or placental vessels. It is 

 true an intricate apparatus consisting of five different classes 

 of vessels is the result, but the play of that apparatus is pre- 

 cisely the same as in the simpler contrivance of fishes. 



Of the Mechanical Force with which these Motions are ac- 

 complished. — The force by which these motions are established 

 is not alone in the proper direction, but also of sufficient in- 

 tensity. Some years ago I made experiments with a view of 

 establishing this point. Some of them are inserted in the 

 Phil. Mag. for Oct. 1838. I found that water, under such 

 circumstances as are here considered, would pass through a 

 piece of peritoneum, though resisted by a pressure of nearly 



