384 COMPARATIVE ANATOMY 



the head. Some of the mixed blood in the right auricle finds its way 

 through the foramen ovale into the left auricle and thence to the left 

 ventricle from which it is pumped to all parts of the body. The blood 

 which is pumped from the right ventricle into the pulmonary artery also 

 finds its way mostly into the dorsal aorta by way of the ductus arteriosus. 

 The pulmonary circulation, it is assumed, is reduced in amount, since the 

 lung sacs are not fully inflated while the embryo is in utero. 



At birth, when the child draws its first breath the lungs become filled 

 with air and expanded, with consequent increase in their capillary circu- 

 lation. As a result, the blood which had been flowing through the ductus 

 arteriosus into the aorta passes to the lungs and returns to the heart by 

 the pulmonary veins. The consequent increase in pressure in the left 

 auricle forces septum I of the auricle against septum II and closes the 

 foramen ovale. (See Fig. 321 and description on page 372.) Thereafter 

 the blood in the left side of the heart is aerated blood, while that of the 

 right side is venous blood. As a result of these changes in circulation 

 the lumen of the ductus arteriosus usually disappears. The walls of 

 the vessel, however, persist as a connective tissue cord, the ligamentum 

 arteriosum of adult anatomy. 



Following the birth of the child the placental circulation stops. That 

 portion of the umbilical vein which extends from the navel to the liver 

 loses its lumen and becomes the ligamentum teres of the liver. The two 

 umbilical arteries are reduced to the lateral umbilical ligaments of the 

 adult body. The ductus venosus of the liver becomes the ligamentum 

 venosum. Some of these changes occur slowly so that in most infants 

 the ductus arteriosus becomes imperforate only at three months. The 

 fusion of the interauricular septa occurs still more slowly, and is complete 

 at three months after birth in only one third of all individuals, and even 

 at maturity is incomplete in a quarter of the population. The failure 

 of the septa to close the foramen ovale results in a "blue" baby, for which 

 the chances of life are small. 



Development of the Lymphatic System. Lymphatic vessels are 

 thin- walled channels which, like arteries and veins, have an endothelial 

 lining and terminate in fine capillaries. In addition to vessels and capil- 

 laries lymphatics have nodular swellings or lymph-nodes which act as 

 "filters of lymph" and centers of lymphocyte formation. Since, how- 

 ever, lymph nodes or glands are supplied with blood capillaries, it cannot 

 be said that they are peculiar to the lymphatic system. 



The origin and differentiation of lymphatics have long been matters of 

 dispute, and some of the issues still remain unsettled. Although lym- 

 phatics at the time of their first appearance as well as in later life are 

 closely associated with veins, it seems to be increasingly evident that they 

 are not formed as outgrowths of the veins. They arise as independent 



