FCETAL CIRCULATION. 439 



the Eustacliian valve. The two ventricles, thus distended, 

 then contract simultaneously. The blood from the right 

 ventricle passes in small quantity to the lungs, the greater 

 part passing through the ductus arteriosus into the descend- 

 ing portion of the arch of the aorta. This duct is short, 

 half an inch in length, and about the size of a goose-quill. 

 The blood from the left ventricle passes into the aorta and 

 goes to the system. The vessels of the head and superior 

 extremities being given off from the aorta before it receives 

 the blood from the ductus arteriosus, .these parts receive al- 

 most the pure blood from the vena cava ascendens, the only 

 mixture with the placental blood being the blood from the 

 lower extremities, the blood from the portal system, and the 

 small amount of blood received from the lungs. After the 

 aorta has received the blood from the ductus arteriosus, how- 

 ever, it is mixed blood ; and it is this which supplies the trunk 

 and lower extremities. This is one of the reasons assigned 

 by physiologists for the greater relative development of the 

 upper parts of the foetus. 



In Fig. 52, which is partly diagrammatic, the foetal circu- 

 lation is illustrated. In endeavoring, in this figure, to give 

 a clear idea of the second circulation, we have not attempted 

 to preserve the exact relations or the relative size of the or- 

 gans. We have endeavored to represent, by dotted lines, the 

 Eustacliian valve, the foramen ovale, and the two auriculo- 

 ventricular orifices. The liver, which is smaller in the dia- 

 gram than it really is, and the bladder are represented by 

 dotted lines. 



There can be no doubt that the foetus derives materials 

 for its nutrition and growth from the placenta, and that this 

 also serves as a respiratory organ. In another volume, 1 under 

 the head of respiration before birth, we have stated that 

 " Legallois frequently observed a bright-red color in the blood 

 of the umbilical vein ; and, on alternately compressing and 

 releasing the vessel, he saw the blood change in color succes- 



1 See vol. i., Respiration, p. 487. 



