296 THE SIXTH DAY. [CHAP. 



dorsal aorta), and is not as yet connected with the right 

 subclavian, R.so. The common carotid of the left side 

 comes off from the fourth arch of the left side ; but since 

 this arch becomes the left subclavian, L.sc. (the connec- 

 tion between the fourth and fifth left arches being 

 obliterated), the portion of the trunk between the fourth 

 arch and the bulbus arteriosus (or as it must now be 

 called the common aortic root) is called the left 

 innominate artery. 



The fourth arch of the right side forms the com- 

 mencement of the great dorsal aorta, and gives off the 

 right subclavian (R.so.) just before it is joined by the 

 fifth arch. 



The fifth arch of each side gives off branches (R.P., 

 L.P.) to the lungs ; their distal continuations, by which 

 these arches are connected with the systemic circulation, 

 though much reduced, are not obliterated. 



The final changes undergone by the arterial system 

 after the commencement of the pulmonary respiration 

 consist chiefly in the complete separation of the pul- 

 monary and systemic circulations. As the branches to 

 the lungs become stronger and stronger, less and less 

 blood from the right ventricle enters into the dorsal 

 aorta ; and the connecting vessels become smaller and 

 smaller. 



Each of these fifth arches from the right ventricle 

 may therefore be considered at about the sixteenth or 

 eighteenth day as divided into two parts, an inner part 

 which connects the heart with the lung, and an outer 

 part which still connects the arch with the main dorsal 

 aorta. As these outer parts become smaller they re- 

 ceive the name of the ' ductus or canales Botalli ' or 



