DEVELOPMENT OF FOURTH DAY 531 



The ventricular septum is completed at about the sixth day, its 

 anterior edge fusing with the posterior edge of the septum which divides 

 the truncus arteriosus into right and left halves. 



The anterior edge of the septum of the truncus arises between the 

 fourth and fifth aortic arches in a manner which causes the blood com- 

 ing from the left side of the truncus (that is, from the left ventricle) 

 to pass through the third and fourth aortic arches, while the blood from 

 the right ventricle passes into the fifth aortic arch. 



About the seventh day the right and left parts of the truncus sepa- 

 rate completely from each other. The right branch remains connected 

 with the fifth aortic arch as the pulmonary trunk, and the left is con- 

 nected with the third and fourth arches as the systemic trunk. 



The ventral ends of the third arches become the subclavian arteries, 

 carrying blood to the anterior appendages, while the dorsal communica- 

 tion between third and fourth arches disappears. 



This means that the blood now passes from the left side of the heart 

 through the third arch to the anterior appendages, and through the 

 fourth arch to the dorsal aorta. 



About the fifth day, the fourth pair of arches are the larger of any 

 arches remaining, the left one, however, becoming smaller and smaller 

 in size until it disappears almost entirely. The right fourth aortic arch 

 grows larger and larger to form the systemic arch of the adult chick. 



It has already been stated that in mammals it is the right arch which 

 disappears, the left alone persisting as the systemic arch (Fig. 309). 



Early on the third day the pulmonary arteries form in the walls of 

 the lungs, and extend toward the fifth arch with which they connect 

 at the ventral ends of these arches. The dorsal end of the fifth arch 

 between the point of union of the pulmonary artery and the dorsal aorta 

 is called the duct of Botallus (Fig. 309). This ductus Botalli offers the 

 blood from the right side of the heart a passage into the dorsal aorta 

 so that little passes through the capillaries. The duct, however, shrivels 

 up at the time of hatching, and becomes entirely closed so that all the 

 blood from the right side of the heart must pass into the pulmonary 

 circulation. It is at this time that the lower portion of the aortic arch 

 becomes the pulmonary artery. 



THE VEINS 



As has been stated several times, the anterior and posterior cardinal 

 veins unite with each other on a side to form the duct of Cuvier and 

 then enter into the meatus venosus. These anterior and posterior cardi- 

 nals bring back the blood to the heart from practically all parts of the 

 body except the digestive organs. 



The anterior cardinals persist as the jugular veins to which the 

 pectoral veins from the anterior appendages soon become joined. From 

 the head and neck the vertebral veins also join the jugulars. 



