CHANGES IX CIRCULATION AT BIRTH 235 



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 plaental 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 into 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, and 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 referred to. It is gradually effected by 

 the union 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 unfre- 

 quently 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 ; 

 this occurs as the malformation attending the morbus cceruleus. 



THE LYMPHATIC SYSTEM. 1 



Lymph-vessels. Little that is quite certain is known regarding the develop- 

 ment of the lymph vascular system. In the lowest vertebrates there is no such 

 system of vessels distinct from the blood vascular system, and the only channels 

 comparable to lymph-vessels are spaces in the connective tissue. In mammals 

 organogenesis is well advanced before there is any sign of walled and valved 

 lymphatic vessels. Up to that stage there are spaces, in certain situations, which 

 no doubt contain lymph, and it has been very generally held that the permanent 

 lymphatics are such spaces round which the connective-tissue cells are arranged to 

 form the walls of the vessels, Awhile the' communication with the veins is secondarily 

 acquired (Gulland, Saxer, Sala). On the other hand, Klein described the lymphatics 

 as developing by the hollowing out of mesenchyme (connective-tissue) cells (vaso- 

 formative cells), which join with one another to form protoplasmic tubes, the walls of 



1 For literature, see Hochstetter, Hertwig III. Th. ii. and iii. p. 165. Also Sabin, Amer. Jour, of 

 Anat. vols. i. iii. and iv. ; Langer (quoted by Sabin), Sitzungsber. d. k. Akad. d. Wissensck. I. Abth. 1868; 

 MacCallum, Archiv f. Anat. u. Phys. Anat. Abth. 1902 ; Lewis, Amer. Jour, of Anat. v. 1905 ; 

 Huntington and McClure, The Anatomical Record, Amer. Jour, of Anat. vi. No. 8, April 1907. 



