PARTURITION 



697 



great vessels which supply the upper parts of the body, and is distributed to 

 the trunk and lower parts of the body. A large portion passes out by way 

 of the two umbilical arteries to the placenta. From the placenta it is returned 

 by the umbilical vein to the under surface of the liver, from which the de- 

 scription started. 



After birth the foramen ovale, the ductus arteriosus, and ductus venosus 

 all close, and the umbilical vessels are tied off, so that the two streams of blood 

 which arrive at the right auricle by the superior and inferior vena cava, re- 

 spectively, thenceforth mingle in this cavity of the heart, and pass into the 



FIG. 505. Dissection of the Lower Half of the Female Mamma During the Period of Lactation, 

 f. In the left-hand side of the dissected part the glandular lobes are exposed and partially un- 

 ravelled, and on the right-hand side the glandular substance has been removed to show the 

 reticular loculi of the connective tissue in which the glandular lobules are placed, i , Upper part 

 of the mammilla or nipple; 2, areola; 3, subcutaneous masses of fat; 4, reticular loculi of the con- 

 nective tissue which support the glandular substance and contain the fatty masses; 5, one of 

 three lactiferous ducts shown passing toward the mammilla, where they open; 6, one of the sinus 

 lactei or reservoirs; 7, some of the glandular lobules which have been unravelled; 7', others massed 

 together. (Luschka.) 



right ventricle, by way of the pulmonary artery to the lungs, and through 

 these, after aeration, to the left auricle and ventricle, to be distributed over 

 the body. 



Parturition. With the implantation of the embryo and the devel- 

 opment of the placenta, the uterus grows rapidly until the end of preg- 

 nancy. The muscles of its walls increase enormously in volume, appar- 

 ently by an increase in the size of the fibers, and the whole structure may 

 become thirty or forty times its size in the resting period. Many changes 

 take place also in other parts of the body, changes which are dependent on 

 the presence of the fetus. Full-term pregnancy occurs when the uterus is 



