560 THE RESPIRATORY EXCHANGE 



tory exchange of his patient indirectly ; he observes the rate and 

 depth of the respiratory movements and the colour of the face 

 and extremities, and hence draws conclusions concerning the 

 intake of oxygen and the output of carbon dioxide. 



Ilfwill, ^therefore, not be amiss to consider here in some detail 

 the means whereby the ventilation of the lungs is regulated. In 

 the first place arises the problem of the first breath, which has long 

 exercised the minds of physicians, lawyers, and theologians, and 

 is still incompletely solved. What are the causes of the first 

 respiration at birth ? The foetus has breathed for months by 

 means of the gaseous exchange which occurs in the placenta l 

 between its blood and the maternal blood ; at birth the infant 

 is obliged to ventilate its lungs in order that it may breathe. The 

 most natural stimulus would, therefore, appear to be want of 

 oxygen. Is lack of oxygen or accumulation of carbon dioxide the 

 stimulus which sets in rhythmic activity the respiratory muscles, 

 which from a condition of apparent inactivity during intrauterine 

 life enter upon a course of rhythmic activity which will cease only 

 with life itself ? Does the stimulus only become effective at 

 birth, or has it been present in a weak form during foetal life ? 

 Have the respiratory muscles been exercised during the last months 

 of intrauterine life, and thereby prepared for the activity they 

 assume at birth, or has the condition of the foetus been one of 

 continual apncea ? 



A cause other than the venosity of the blood has been strongly 

 advocated by some observers. During intrauterine life the foetus 

 lies in the warm amniotic fluid, which shields both it and the 

 mother from injury ; at birth the infant is exposed and is cooled 

 even in a warm room by the evaporation of the liquid covering 

 its body ; this cooling is said to be the stimulus which starts the 

 first breath. An undue importance has been given to this 

 supposed cause. An infant born in the tropics draws its first 

 breath as readily as one born in a temperate climate or in the 

 arctic regions. Numerous cases, moreover, are on record which 

 show that breathing by the lungs may commence when only the 

 head is born, or even when the head is in the genital canal. Infants 

 and young animals have been born and have drawn their first 

 breaths inside the intact foetal membranes. It is equally certain 



1 See page 569. 



