302 RESPIRATION 



It is also apparent that the respiratory center is dependent on the 

 character of the blood supply, both as regards quantity or quality of the 

 blood. It has been shown that the presence in the blood of the products 

 of vigorous muscular metabolism will greatly increase the irritability of the 

 respiratory center, even if the blood itself be not particularly venous in 

 character as regards its gaseous content. 



The Establishment of Respiratory Movements at Birth. From 

 the preceding paragraph it appears that the regulation of the respiratory 

 movements is normally due to the automaticity of the respiratory center as 

 influenced, first, by the blood flowing through it and, second, by the afferent 

 nerve impulses which reach the center. The fetus in the womb is supplied 

 by arterial blood from the blood vessels of the mother. The fetus does not 

 ordinarily give respiratory movements before birth, but it may be made to 

 do so by experimental procedure. At birth the placental circulation is sud- 

 denly interrupted, and the blood rapidly increases in venosity until the skin, 

 lips, and mucous membranes are very cyanotic in appearance. It is at this 

 time that the respiratory center begins its rhythmic discharges, being aroused 

 by the direct stimulating effects of the great excess of carbon dioxide in the 

 strongly venous blood. It is more than possible that the irritability of the 

 center is also increased by the stimulation of the skin by the air, the contact 

 with clothing, and the hands of the nurse. We have already seen that 

 cutanous stimulation leads to increase in both respiratory rate and ampli- 

 tude even in the adult. The primary stimulus for the establishment of the 

 respiratory rhythm at birth, then, is the venosity of the blood, but this 

 cause is supported by the general reflexes which reach the respiratory 

 center. 



Certain Special Types of Respiration. Whatever the exact quality 

 of the venous blood which excites the respiratory center to produce normal 

 respirations, there can be no doubt that, as the blood becomes more and 

 more venous from obstruction to the entrance of air into the lungs or from 

 the blood not taking up from the air its usual supply of oxygen, the respi- 

 ratory center becomes either less or more active and excitable. Conditions 

 ensue which have received the names Apnea, diminished breathing; Hyper- 

 pnea, excessive breathing; Dyspnea, difficult breathing; and Asphyxia, 

 suffocation. 



Apnea. This is a condition of diminished respiratory movement. When 

 we take several deep inspirations in rapid succession by voluntary effort, 

 we find that we can do without breathing for a much longer time than usual; 

 in other words, several rapid respirations seem to inhibit for a time normal 

 respiratory movements. The reason for this partial cessation of respira- 

 tion, or apnea, is not that we overcharge our blood with oxygen, as was once 

 thought, for Hering has shown that animals in a condition of apnea may 

 have less oxygen in their blood than in a normal state, although the carbon 



