THE LYMPHATIC SYSTEM. 157 



CHANGES IN THE CIBCULATION AT BIKTH. 



The changes which occur in the organs of circulation and respiration at birth, 

 and which lead to the establishment of their permanent condition, are more 

 immediately determined by the inflation of the lungs with air in the first 

 respiration, the accompanying rapid dilatation of the pulmonary blood-vessels 

 with a greater quantity of blood, and the interruption to the passage of blood 

 through the placental circulation. These changes are speedily followed by 

 shrinking and obliteration of the ductus arteriosus, and of the hypogastric arteries 

 from the iliac trunk to the place of their issue from the body by the umbilical 

 cord ; by the cessation of the passage of blood through the foramen ovale, and 

 somewhat later by the closure of that foramen, and by the obliteration of the 

 umbilical vein as far as its entrance into the liver, and of the ductus venosus behind 

 that organ. 



The process of obliteration of the arteries appears to depend at first mainly on 

 the contraction of their coats, but this is very soon followed by a considerable 

 thickening of their substance, reducing rapidly their internal passage to a narrow 

 tube, and leading in a short time to final closure, even although the vessel may not 

 present externally any considerable diminution of its diameter. It commences at 

 birth, and is perceptible after a few respirations have occurred. It makes rapid 

 progress in the first and second days, and by the third or fourth day the passage 

 through the umbilical arteries is usually completely interrupted. The ductus 

 arteriosus is rarely found open after the eighth or tenth day, and by three weeks 

 it has in almost all instances become completely impervious. 



The process of closure in the veins is slower ; but they remain empty of blood 

 and collapsed, and by the sixth or seventh day are generally closed. 



Although blood ceases at once to pass through the foramen ovale from the 

 moment of birth, or as soon as the left auricle becomes filled with the blood 

 returning from the lungs, arid the pressure within the two auricles tends to be 

 more equalised during their diastole, yet the actual closure of the foramen is more 

 tardy than any of the other changes now referred to. It is gradually effected by 

 the union of the forepart of the valve of the fossa ovalis with the margin of the 

 limbus of Vieussens on the left side ; but the crescentic margin is generally perceptible 

 in the left auricle as a free border beyond the place of union, and not unfrequently 

 the union remains incomplete, so that a probe may be passed through the reduced 

 aperture. In many cases a wider aperture remains for more or less of the first year 

 of infancy, and in certain instances there is such a failure of the union of the valve 

 as to allow of the continued passage of venous blood, especially when the circulation 

 is disturbed by over-exertion, from the right to the left auricle, as occurs iii the 

 malformation attending the morbus coeruleus. 



THE LYMPHATIC SYSTEM. 



The development of the lymphatic system has been studied in the chick by Budge. Here 

 there exist a network of lymphatics agreeing- in their general distribution with the blood- 

 vessels in the vascular area. This is the first lymphatic circulation ; a second one is formed 

 later, corresponding with the allantoic circulation. The lymphatics of both systems communi- 

 cate with the coelom, but only those of the second circulation communicate with veins. The 

 lymphatics lie in the vascular area above and close to the blood-vessels, but are separated 

 near the embryo by a layer of mesoblast continuous with the splanchnopleure. 



The method of development of lymphatic vessels and lymphatic glands, is dealt with in 

 the chapter on Histology. 



