274 PHYSIOLOGY CHAP. 



child. When the placental exchange of gases between the foetus and 

 the mother is interrupted by the detachment of the placenta, blood 

 sufficiently arterial ceases to reach the spinal bulb, and when the 

 venosity of the blood supplied to it reaches a certain point, the 

 respiratory centre begins to act, and initiates the alternate 

 inspiratory and expiratory movements. The air passes through 

 the nose and mouth into the air passages, distends the pulmonary 

 vesicles, and the blood of the pulmonary artery, of which the 

 greater part went to the aorta through the ductus arteriosus, now 

 goes to the pulmonary capillaries, through which begins the 

 gaseous exchange between the surrounding air and the blood 

 circulating through the lungs 



Owing to the diminished resistance to the passage of the blood 

 through the pulmonary circulation brought about by the expansion 

 of the thorax, a greater quantity of blood passes this way instead 

 of going through the ductus arteriosus Botalli. "With each 

 successive respiratory movement the quantity of blood which 

 passes into the capillaries of the lungs from the pulmonary artery 

 increases, and the current which passes from this artery through 

 the ductus arteriosus diminishes proportionately, until it finally 

 ceases altogether. Consequently this duct is obliterated soon 

 after birth, as was observed by P. Heger and Marique in forty-seven 

 children, thus confirming the previous observations of Heller. 

 According to Costa Alvarenga (1870), the transformation of the 

 ductus arteriosus into a fibrous cord is completed at from the 

 second to the fifth month ; Walkhoff (1879), however, who studied 

 its histological modifications, considers that the transformation 

 is accomplished as early as the twentieth day after birth. It is 

 facilitated by the speedy formation of an occluding thrombus, and 

 is completed later by proliferation of the intima, in which even 

 before birth the sub-endothelial layer is unusually developed. 



Corresponding to the greater flow through the pulmonary 

 artery there is a steadily increasing flow of blood towards -the 

 pulmonary capillaries where it changes from venous into arterial 

 blood, returning to the left auricle through the pulmonary veins. 



At the same time, owing to the cessation of the flow of blood 

 from the umbilical vein through the ductus Arantii, the outflow 

 from the inferior vena cava is diminished and the blood in the 

 right auricle, meeting with little resistance in the direction of the 

 right ventricle, which now rapidly discharges its contents into 

 the pulmonary artery, finds in the left auricle, now filled with 

 blood from the pulmonary veins, an obstacle to its passage through 

 the foramen ovale. The increase of pressure in the left auricle 

 creates a condition favourable to the closure of the foramen. It 

 is obliterated by means of a valve which makes its appearance in 

 the human embryo towards the eighth week, and develops in 

 front of the Eustacliian valve, of which it seems to be a prolonga- 



