246 HUMAN PHYSIOLOGY 



is that which has been improved in the placenta by 

 effecting exchanges with the blood in the uterine vessels. 

 This is introduced into the veins of the child. The 

 stream is sufficiently abundant to keep the average 

 composition of the blood suitable for nutrition. The 

 placenta is more than a lung; it is the seat of absorption 

 . of organic food and serves for the unloading of wastes. 

 It takes the place, for the time, of the intestine and the 

 kidneys. 



At birth, the placental circulation is interrupted. 

 The child is cast upon its own resources and it must begin 

 to use its lungs or perish from suffocation. The stimulus 

 to the medulla to initiate breathing may be due to the 

 rising percentage of carbon dioxid in the blood but it is 

 likely to be reinforced by the effect of strange contacts 

 and an unwonted temperature at the surface of the body. 

 When the first breath is taken the larger respiratory 

 passages are full of liquid. This retreats from the nostrils 

 as the air enters and the compressed system is opened 

 out much as one expands a Christmas bell of tissue 

 paper. The same movement that dilates the air spaces, 

 rounds the flattened capillaries of the lungs and im- 

 mediately a large share of the blood pumped out by the 

 right ventricle takes its course through them. 



The possibility of some crossing over between the two 

 sides of the heart remains for a time but the ductus 

 arteriosus and the foramen ovale are quite rapidly closed. 

 In a few days, in normal cases, no blood can escape 

 going through the lungs after it has journeyed through 

 the systemic circulation. Occasionally there is an un- 

 fortunate failure to close the embryonic by-passes and 

 so much venous blood enters the arterial system that a 

 " blue baby " is the result. In such instances the chances 

 of survival are precarious and there is more or less marked 

 malnutrition. 



