RESPIRATION 437 



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 

 431, 433 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 of 

 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 sudden 

 inspiratory movement. 



MODIFICATIONS OF THE RESPIRATORY RHYTHM 



The rhythmic character of the respiratory movements is materially 

 modified by a change in the quantitative and qualitative composition of the 

 air and blood as well as by changes of a pathologic nature of the respiratory 

 apparatus itself. 



Eupnea. So long as the air retains its normal composition and the 

 respiratory mechanism its structural integrity, so long do the respiratory 

 movements exhibit a normal rhythm and frequency. To the condition of 

 easy tranquil breathing the term eupnea is given. In this condition the 

 percentages of oxygen and carbon dioxid in the blood are such as to favor at 

 least the rhythmic discharge of nerve impulses from the inspiratory center 

 to the inspiratory muscles, of sufficient energy and frequency for the main- 

 tenance of normal respiration. 



Hyperpnea. The normal rate of the respiratory movements is increased 

 by a rise in body-temperature, by active exercise, and by emotional states. 

 Whatever the cause, the increase in rate and probably in depth is termed 

 hyperpnea. 



Febrile states characterized by a rise in the temperature of the blood 

 increase considerably the respiratory activity. This is due in all probability 

 to a warming of the inspiratory center, in consequence of which its ir- 

 ritability is heightened; for surrounding the carotid arteries with warm tubes 

 and heating the blood on its way to the medulla has the same effect. It is 

 also possible, however, that the high temperature of febrile conditions may 

 interfere with the absorbing power of hemoglobin, and thus by diminish- 

 ing the quantity of oxygen absorbed lead to more frequent respirations. 

 To the hyperpnea induced by heat the term thermo-polypnea is frequently 

 given. 



Muscle activity, especially if it is violent and indulged in by those unac- 

 customed to exercise, is generally followed by increased rate and depth of 

 breathing, and not infrequently it is attended with such extreme difficulty 

 that the condition approximates that of dyspnea. This condition is attrib- 



