THE CIRCULATION OF THE BLOOD 185 



opens into the right auricle of the heart. From the 

 right auricle the blood passes over the Eustachian 

 valve into the right ventricle, then from the latter 

 into the pulmonary artery. The lungs of the fetus 

 being inactive, require only enough blood to develop 

 and nourish them, this quantity of blood is conveyed 

 by the pulmonary arteries and returned to the pul- 

 monary veins to the left auricle; the greater quantity 

 of blood from the right ventricle passes through the 

 ductus arteriosus (a small vessel connecting the pul- 

 monary artery with the aorta) into the beginning of 

 the descending aorta, where it becomes mixed with 

 the blood from the left ventricle. 



The general distribution of blood through the body 

 of the fetus is similar to the adult system, with the 

 exception of the above-mentioned differences and the 

 return of waste materials from the fetus to the placenta 

 by way of the hypogastric arteries, which are branches 

 of the internal iliacs and join the umbilical vein at 

 the umbilicus (navel) to assist in forming the umbilical 

 cord of the fetus. 



The Changes in the Circulatory Apparatus following 

 Birth. With the commencement of normal respiration 

 (the umbilical cord being ligated), the placental circu- 

 lation is cut off. Anywhere within ten days after 

 birth the foramen ovale closes and may continue as 

 a slight opening and not be injurious to health until 

 the final closure. When the foramen does not close, 

 the skin, etc., becomes cyanotic, due to the mixing of 

 the arterial and venous blood the so-called blue baby. 



The umbilical vein, ductus venosus, and arteriosus 

 atrophy, and also the hypogastric arteries. Thus the 

 infant vascular apparatus assumes the normal course 

 as described in the adult. 



Structure and Functions of the Arteries, Veins, and 

 Capillaries. The Arteries. These are closed tubes 

 which convey the blood and keep it in circulation 

 from the heart cavities throughout the body. 



