Chap. XII] GENERAL CIRCULATION OF BLOOD 227 



2. As respiration commences, the blood traverses the pul- 

 monary arteries, and then returns to the heart by the pulmonary 

 veins ; this raises the blood pressure in the left auricle, and causes 

 the valve over the foramen ovale to close. 



3. The blood in the ductus arteriosus clots, the clot organizes, 

 and the ductus arteriosus becomes a fibrous cord. 



4. The blood in the hypogastric arteries also clots, the clots 

 organize and these vessels become obliterated. 



