43 2 TEXT-BOOK OF PHYSIOLOGY 



center becomes more irritable and discharges its energy more frequently 

 and to a greater degree as shown by the increase in the rate and the depth 

 of the inspiratory movement. 



The same observers have also shown that when an individual is caused 

 to inhale air the percentage of the oxygen of which had been reduced from 

 20 to 13 and therefore to about 8 per cent, in the alveolar air instead of about 

 15 per cent, no particular change in either the frequency or the depth of 

 the inspiratory movements was noticed, but when the percentage of the 

 oxygen was lowered below this amount the inspiratory center became more 

 irritable as shown by an increase in the rate and depth of the inspiratory 

 movement. As a rule the oxygen percentage in the alveolar air must be 

 reduced fully one-half and thereby the percentage and pressure of the oxygen 

 in the arterial blood fully one-third before the respiratory center is stimulated 

 to increased activity. A reason assigned for this result is the presence in 

 the blood of some non-oxidized metabolic product, probably lactic acid, that 

 is acting as the stimulus. All recent experimental work confirms the view 

 that the specific stimulus to the inspiratory center is the normal pressure of 

 the CO 2 in the blood and so responsive is it to this agent that an increase 

 in even 0.2 per cent, in the alveolar air is sufficient to almost double the 

 respiratory ventilation. 



The Establishment of Respiration after Birth. Previous to birth 

 the exchange of the gases which constitutes the respiratory activity in the 

 mammalian fetus, takes place in the placenta. The venous blood is carried 

 by the umbilical artery to this organ in which the blood of the fetus comes 

 into close relation with the blood of the mother, the two fluids being separated 

 only by an extremely thin partition. The venous blood while passing 

 through the placenta yields up its carbon dioxid to and receives oxygen 

 from the maternal blood; after this exchange of gases the now-arterialized 

 blood is returned to the fetus by the umbilical veins. 



Immediately after birth and the detachment of the placenta, this method 

 of gaseous exchange is abolished and if the life of the child is to be main- 

 tained the respiratory movement must be established. The cause of the 

 first inspiration, therefore, must be associated with an increase in the per- 

 centage of carbon dioxid or a decrease in the percentage of oxygen in the 

 blood. In accordance with statements in foregoing paragraphs the former 

 condition is more likely to be the efficient cause. The rapid accumulation 

 of carbon dioxid with its increasing pressure in the inspiratory center so 

 raises its irritability as to lead to a discharge of nerve impulses which are 

 conducted to the inspiratory muscles and cause their contraction. With 

 the first inspiration thus established the nerve mechanism described, pages 

 424, 426 comes into play. 



Inasmuch as cold water applied to the skin of the adult profoundly ex- 

 cites at times the inspiratory center it has been assumed that an additional 

 factor leading to an excitation of the inspiratory center is the rapid cooling oi 

 the surface of the child by the evaporation of the amniotic fluid from the 

 surface of the skin. The nerve impulses thus developed are transmitted 

 through cutaneous nerves to the inspiratory center. This assumption is 

 somewhat strengthened by the fact that in delayed inspiration the stimula- 

 tion of the skin by the application of cold water frequently leads to a sudder 

 inspiratory movement. 



