792 



DEVELOPMENT 



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 description 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, 

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

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



FIG. 507. Dissection of the Lower Half of the Female Mamma During the Period of 

 Lactation. . In the left-hand side of the dissected part the glandular lobes are exposed 

 and partially unravelled, 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; a, areola; 3, subcutaneous masses 

 of fat; 4, reticular loculi of the connective 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 lartei or reservoirs; 7, some of the glandular 

 lobules which have been unravelled; 7', others massed together. (Luschka.) 



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

 the body. 



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

 ment of the placenta, the uterus grows rapidly until the end of pregnancy. 

 The muscles of its walls increase enormously in volume, apparently 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 



