CH. LIX.] PARTURITION 893 



superior vena cava. It might be naturally expected that the two 

 streams of blood would be mingled in the right auricle, but such is 

 not the case, or only to a slight extent. The blood from the superior 

 vena cava the less pure fluid of the two passes almost exclusively 

 into the right ventricle, through the auriculo-ventricular opening, just 

 as it does in the adult ; while the blood of the inferior vena cava is 

 directed by the fold of the lining membrane of the heart, called the 

 Eustachian valve, through the foramen ovale into the left auricle, 

 whence it passes into the left ventricle, and out of this into the aorta, 

 and thence to all the body, but chiefly to the head and neck. The 

 blood of the superior vena cava, which, as before said, passes into the 

 right ventricle, is sent out from there in small amount through the 

 pulmonary artery to the lungs, and thence to the left auricle, by the 

 pulmonary veins, as in the adult. The greater part, however, does 

 not go to the lungs, but instead, passes through a canal, the ductus 

 arteriosuA, leading from the pulmonary artery into the aorta just below 

 the origin of the three great vessels which supply the upper parts of 

 the body ; and there meeting that part of the blood of the inferior 

 vena cava which has not gone into these large vessels, it is distributed 

 with it to the trunk and other parts a portion passing out by way 

 of the two umbilical arteries to the placenta. From the placenta it 

 is returned by the umbilical vein to the under surface of the liver, 

 from which the description started. 



PARTURITION 



During pregnancy the uterus and its contents increase in size, 

 and we have already alluded to the changes in its mucous membrane 

 or decidua, and the formation of the placenta ; the principal factor 

 in the distension of the uterus is the accumulation of the amniotic 

 fluid. The muscular wall of the uterus also hypertrophies ; this is 

 in part due to the formation of new muscular fibres, and in part of 

 the increase in size of the pre-existing muscular fibres. The 

 muscular wall is one of immense strength. 



The foetus " comes to term " in the human subject on the tenth 

 menstrual epoch after conception; this averages about 280 days 

 after the last menstruation. Delivery is the result of uterine 

 contractions or "labour pains"; the liquor amnii is thus forced 

 downward and presses the membrane formed by the fused amnion 

 and chorion through the cervix of the uterus which is gradually 

 distended. When the distension is sufficient the membrane ruptures, 

 and the amniotic fluid escapes. The os is then fully distended, and 

 the foetal head enters the pelvis ; the pains become more frequent 

 and energetic, and the voluntary muscles of the abdomen are brought 

 into play, so that ultimately the new-born child is expelled to the 



