852 THE NUTRITION OF THE EMBRYO. 



to the air, or to cold ; but these are subordinate, not essential. A retarded 

 first breath may be hurried on by dashing water on the face of the new- 

 born infant ; but, on the other hand, the foetus, upon the cessation of the 

 placental circulation, will 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. 



794. Before this first breath is taken the pulmonary alveoli contain 

 no air, and the lungs when thrown into water sink at once ; they are then 

 said to be " atelectatic." After the first breath, the alveoli contain air and 

 the lungs float when thrown into water. A striking difference, however, 

 exists between the lungs of a newborn infant and those of an older person. 

 When the pleural cavity of the former is opened, the lungs do not collapse, 

 no air is driven out by the trachea ; that partial distention of the lungs, and 

 negative thoracic pressure, appears not to be established immediately upon 

 birth. That portion of the residual air in the lungs of the adult which, 

 remaining after the most forcible expiration, is still driven from the lungs 

 upon the pleural cavity being laid open, and which might be called " col- 

 lapse air," is wanting in the newborn infant. When the change from one 

 condition to the other is effected is not at present known ; it may possibly 

 arise from the growth of the chest outstripping that of the lungs. 



When the first breath is taken, as under normal circumstances it is, with 

 free access to the atmosphere, the lungs become filled with air, the scanty 

 supply of blood which at the moment was passing from the right ventricle 

 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. 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 cur- 

 rent through the ductus venosus from the umbilical vein having ceased, the 

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

 auricle finding little resistance in the direction of the ventricle, which now 

 readily discharges its contents into the pulmonary artery, but finding in the 

 left auricle, which is continually being filled from the lungs, an obstacle to 

 its passage through the foramen ovale, ceases to take that course. Any re- 

 turn of blood from the now vigorous and active left auricle into the right 

 auricle is prevented by the valve which, during the later stages of intra- 

 uterine life, has been growing up in the left auricle over the foramen ovale. 

 At birth the edge of this valve is to a certain extent free, so that, in case of 

 an emergency, as when the pulmonary circulation is obstructed, a direct 

 escape of blood into the left auricle from, the over-burdened right auricle 

 can take place. Eventually, in the course of the first year, adhesion takes 

 place, and the separation of the two auricles becomes complete. With its 

 larger supply of blood and greater work the left ventricle acquires the 

 greater thickness characteristic of it during life. Thus the fcetal circula- 

 tion, in consequence of the respiratory movements to which its interruption 

 gives rise, changes its course into that characteristic of the adult. 



