CHAP. HI.] THE NUTRITION OF THE EMBRYO. 679 



mtra-uterine life the amount of oxygen in the blood passing from 

 the aortic arch to the medulla oblongata is sufficient to prevent 

 any inspiratory impulses being originated in the medullary 

 respiratory centre. This during the whole period elapsing between 

 the date of its structural establishment, or rather the consequent 

 full development of its irritability, and the epoch of birth, remains 

 dormant; the oxygen-supply to the protoplasm of its nerve-cells 

 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 interrupted by separation of the 

 placenta or by ligature of the umbilical 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 medulla oblongata, 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 respiratory centre 

 may be assisted by adjuvant impulses reaching the centre along 

 various afferent nerves, such as those started by exposure of the 

 body 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. 



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 new-born 

 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 distension of the lungs, and 

 negative thoracic pressure, which we studied (p. 367) in treating of 

 respiration, appears not to be established immediately upon birth. 

 That portion of the residual air (p. 315) 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 'collapse air,' is wanting in the new-born 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, and 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 



