936 GENERATION. 



little mixture of the two colors in the passage of the injected material through the right 

 auricle. 



The blood poured into the left auricle from the vena cava ascendens through the fora- 

 men ovale passes from the left auricle into the left ventricle. The left auricle and the 

 left ventricle also receive a small quantity of blood from the lungs, by the pulmonary 

 veins. Thus the left ventricle is filled. At the same time, the right ventricle is filled 

 with blood which has passed through the right auricle, in front of the Eustachian 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 descending 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 ven- 

 tricle 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 almost exclusively 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. 311, which is partly diagrammatic, the foetal circulation 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 organs. We have 

 endeavored to represent, by dotted lines, the Eustachian valve, the foramen ovale, and 

 the two auriculo-ventricular orifices. The liver, which is smaller in the diagram 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 chapter, 

 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 com- 

 pressing and releasing the vessel, he saw the blood change in color successively from red 

 to dark and from dark to red." This difference in color between the blood of the umbili- 

 cal arteries and of the umbilical vein has, however, been denied by some authors, who 

 state that all of the foetal blood, while it is of nearly a uniform color, is lighter than the 

 venous blood of the adult ; but Dalton, in a direct observation upon a cat, " nearly arrived 

 at the term of pregnancy," noted that " the difference in color between the umbilical 

 arteries and veins was very distinct. They were both dark, but the color of the veins 

 was very decidedly more ruddy than that of the arteries." By means of the spectroscope, 

 Zweifel has demonstrated the presence of oxyhsemaglobine in the blood of the umbilical 

 vessels, showing that it contains oxygen, which must be derived from the maternal blood 

 circulating in the placental sinuses. 



There are numerous observations showing that the foetus in utero makes respiratory 

 efforts when the umbilical vessels are compressed. We believe that these, as well as the 

 first respiration after birth, are due to a want of oxygenated blood in the medulla oblon- 

 gata, and we think that we have demonstrated this fact by experiments. This point has 

 already been elaborately discussed in another chapter. If our experiments and the deduc- 

 tions drawn from them be correct, there can be no doubt with regard to the respiratory 

 function of the placenta, although, as far as we know, there has never been an accurate 

 comparison of the gases contained in the blood of the umbilical arteries and the umbili- 

 cal vein. 



The Third, or Adult Circulation. When the child is born, the placental circulation 

 is suddenly arrested. After a short time, the sense of want of air becomes sufficiently 



