430 GENERATION. 



of Botal, which persists during the whole of foetal life. At 

 the anterior edge of the opening of the vena cava ascendeiis 

 into the right auricle, there is a membranous fold, which 

 projects into the auricle. This is the valve of Eustachius, 

 and it divides the right auricle incompletely into two por- 

 tions. 



During the sixth week, the heart is vertical and situ- 

 ated in the median line, with the aorta arising from the cen- 

 tre of its base. At the end of the second month, it is raised 

 up by the development of the liver, and its point presents 

 forward. During the fourth month, it is twisted slightly 

 upon its axis, and the point presents to the left. At this 

 time, the auricular portion is larger than the ventricles ; but 

 the auricles diminish in their relative capacity during the lat- 

 ter half of intra-uterine life. The pericardium makes its ap- 

 pearance during the ninth week. 1 



Early in intra-uterine life, the relative size of the heart is 

 very great. At the second month, its weight, in proportion 

 to the weight of the body, is 1 to 50. This proportion, how- 

 ever, gradually diminishes until, at birth, the ratio is 1 to 120. 

 The proportionate weight in the adult is about 1 to 160. 2 

 During about the first half of intra-uterine life, the thickness 

 of the two ventricles is nearly the same ; but, after that time, 

 the relative thickness of the left ventricle gradually increases. 



Peculiarities of the Foetal Circulation. In studying the 

 complete course of the blood in the foetus, which constitutes 

 the second, or the placental circulation, we note peculiarities 

 in two portions of the circulatory system. In the one, a pe- 

 culiar arrangement is necessitated by the passage of blood to 

 and from the placenta ; and in the other, the character of the 

 blood coming from the placenta necessitates a peculiar ar- 

 rangement of the heart and the great vessels. 



The branches from the internal iliac arteries, which pass 



1 BURDACH, Traite de physiologic, Paris, 1838, tome iii., p. 515. 



2 QUAIN, Elements of Anatomy, London, 1867, vol. i., p. 32Y. 



