604 DEVELOPMENT [CHAP. XLII. 



line of the body. The anterior pair are the jugular veins, and the 

 posterior pair are the cardinal veins, which carry back the blood 

 from the Wolffian bodies and hinder parts of the embryo. Besides 

 these trunks, however, there is a single one below, which receives 

 the blood from the omphalo-mesenteric veins, and into this trunk 

 the umbilical veins open subsequently. It becomes eventually 

 converted into the inferior vena cava. 



The pulsations of the heart commence before any cavity can be 

 observed in the mass of embryonic cells of which it at first consists. 

 Prevost and Lebert have observed the contractions before the 

 development of any tissue distinctly muscular a statement which 

 we can confirm from observations upon the heart of the young 

 field-snake (coluber natrix). Bischoff and Vogt also testify to the 

 very early occurrence of pulsations. 



In the human subject, about the fourth week, the septum be- 

 tween the ventricles commences to be formed. This is completed 

 by the termination of the eighth week. The auricular septum, 

 however, remains incomplete throughout f ratal life. The circu- 

 lation in the foetus, and the peculiarities of the foetal heart, have 

 been already described in page 348 of the present volume. 



Aortic Arches. In fishes, the vessel continuous with the bulbus 

 arteriosus gives off on either side large branches, which are dis- 

 tributed to the gills ; from these organs the blood is collected 

 by small vessels, which ultimately re-unite to form a large trunk 

 corresponding to the aorta, which lies immediately in front of the 

 spine. In the early embryos of all vertebrate animals, similar 

 branches, called aortic arches, may be seen; and these unite at the 

 back of the visceral cavity, to form the descending aorta. They 

 are visible in the chick about the fortieth hour, according to Dr. 

 Allen Thomson. 



In birds there are at first six aortic arches ; but, as developmenl 

 proceeds, the number become less. In mammalia the arches 

 soon diminish to three. One becomes the arch of the aorta, and 

 the other two are the ductus arteriosus of the pulmonary artery, of 

 which the right soon disappears, so that at length only two arches 

 remain one from the right, and the other from the left ventricle. 

 The anterior part of the arch from the former becomes the trunk 

 of the pulmonary artery ; while the cavity of the posterior portioi 

 (ductus arteriosus), which leads into the aorta, gradually becomes 

 obliterated, and soon after birth nothing remains of it but a fibrous 

 cord, between the aorta and pulmonary artery, which marks its 

 original position throughout life. 



