CHAP, ii.] PREGNANCY AND BIRTH. 393 



of its irritability and the epoch of birth, remains dormant ; the 

 oxygen-supply to its substance is never brought so low as to set 

 going the respiratory molecular explosions. As soon however as 

 the intercourse between the maternal and umbilical blood is in- 

 terrupted by separation of the placenta or by ligature of the um- 

 bilical cord, or when, as by the death of the mother, the umbilical 

 blood ceases to be replenished with oxygen by the maternal blood, 

 or when in any other way blood of sufficiently arterial quality 

 ceases to find its way by the left ventricle to the bulb, the supply 

 of oxygen in the respiratory centre sinks, and when the fall has 

 reached a certain point an impulse of inspiration is generated and 

 the foetus for the first time breathes. This action of the respira- 

 tory centre may be assisted by adjuvant impulses reaching the 

 centre along various afferent nerves, such as those started by ex- 

 posure of the body to the air, or to cold ; but these are sub- 

 ordinate, not essential. A retarded first breath may be hurried 

 on by dashing water on the face of the new-born infant ; but so 

 long as the placental circulation is intact, stimulation, even varied 

 and strong, of the fetal skin, though it may give rise to reflex 

 movements of the limbs and other parts will not call forth a breath ; 

 whereas, on the other hand, upon the cessation of the placental 

 circulation, the foetus may make its first respiratory movements 

 while it is still invested with the intact membranes and thus 

 sheltered from the air and indeed from all external stimuli. 



964. When the first breath is taken, as under normal cir- 

 cumstances it is, with free access to the atmosphere, and the lungs 

 become inflated with air (we dwelt in dealing with respiration, 

 326, on some features of this first breathing), the scanty supply 

 of blood which at the moment was passing from the right ven- 

 tricle along the pulmonary artery returns to the left auricle 

 brighter and richer in oxygen than ever was the foetal blood 

 before. With the diminution of resistance in the pulmonary 

 circulation caused by the expansion of the thorax, a larger 

 supply of blood passes into the pulmonary artery instead of 

 into the ductus arteriosus, and this derivation of the contents 

 of the right ventricle increasing with the continued respiratory 

 movements, the current through the latter canal at last ceases 

 altogether, and its channel shortly after birth becomes obliterated. 

 The obliteration is ultimately secured by proliferation of the 

 internal coat, in which even before birth the sub-epithelial layer 

 is unusually developed, a thrombus ( 33) at times helping, but 

 before this takes place the closure seems to be assisted by 

 the mechanical arrangements of the parts. Corresponding to 

 the greater flow into the pulmonary artery, a larger and larger 

 quantity of blood returns from the pulmonary veins into the 

 left auricle. At the same time the current through the ductus 

 venosus from the umbilical vein having ceased, the flow from 

 the inferior cava has diminished; and the blood of the right 



