1134 THE NUTRITION OF THE EMBRYO. [BOOK iv. 



distinctly arterial (but still mixed) blood is driven by the left 

 ventricle to the cerebral structures, which, we may conclude, 

 have more need of oxygen than have the other tissues. Con- 

 trary to what takes place afterwards, the work of the right 

 ventricle is in the fcetus greater than that of the left ; and, ac- 

 cordingly, that greater thickness of the left ventricular walls, 

 so characteristic of the adult, does not become marked until 

 close upon birth. 



704. In the later stages of pregnancy the mixture of the 

 various kinds of blood in the right auricle increases prepara- 

 tory to the changes taking place at birth. But during the 

 whole time of intra-uterine life the amount of oxygen in the 

 blood passing from the aortic arch to the brain is sufficient to 

 prevent any inspiratory impulses being originated in the bul- 

 bar 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 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 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 replen- 

 ished 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 t)f 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 

 maybe 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 essen- 

 tial. 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 foetal 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 fcetus 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. 



705. 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 res- 

 piration, 257, on some features of this first breathing), the 

 scanty supply of blood which at the moment was passing from 

 the right ventricle along the pulmonary artery returns to the 







