INNERVATION OF THE RESPIRATORY MOVEMENTS. 643 



of the carbon dioxid falls below a certain value, estimated by 

 Zuntz as lying between 19 and 24 mms., no stimulation occurs, 

 the center is in a condition of apnea and respiratory movements 

 cease. Under special circumstances other substances may occur 

 in the blood which act as chemical stimuli to the center, for example, 

 the acid products formed during muscular contractions (see p. 648). 

 The Cause of the First Respiratory Movement. The mam- 

 malian fetus under normal conditions makes no respiratory move- 

 ments while in utero. After birth and the interruption of the pla- 

 cental circulation the first breath is taken. The cause of this 

 sudden awakening to activity on the part of the respiratory center 

 must be closely connected, if not identical with, the cause of the 

 automatic activity of the center throughout life. Two or perhaps 

 three views have been held regarding its immediate cause: (1) 

 That it is due to the increased venosity of the blood brought about 

 by the interruption of the placental circulation; (2) that it is due to 

 stimulation of the skin by handling, drying, etc. ; (3) that it is due 

 to a combination of these causes. Preyer has shown that stimula- 

 tion of the skin of the fetus while in utero and with the placental 

 circulation intact sufficies to cause respiratory movements. Cohn- 

 stein and Zuntz* have shown that interruption of the placental 

 circulation while the fetus is kept bathed in the amniotic liquid also 

 brings about respirations. Since both of these events occur normally 

 at birth, we may believe that each aids in causing the first respira- 

 tion, and indeed it may be necessary at times deliberately to in- 

 crease the stimulation of the skin in order to bring on respiratory 

 movements. If the two causes, stimulation through the nerves and 

 stimulation through the blood, normally co-operate, it may, how- 

 ever, be said that the essential cause, according to the theory 

 adopted in the preceding paragraphs, lies in the greater venosity of 

 the blood, that is, the increased tension of the carbon dioxid follow- 

 ing interruption of the placental circulation. During the intra- 

 uterine period it is evident that the fetal blood is aerated so well 

 by exchange with the maternal blood that it does not act as a 

 stimulus to the fetal respiratory center. The fetus is, physiolog- 

 ically speaking, in a condition of apnea. Since the maternal blood 

 acts upon the respiratory center of the mother, while the fetal 

 blood which exchanges gases with it does not act on its own respira- 

 tory center, it follows that the fetal respiratory center possesses a 

 lower degree of irritability than that of the mother. 



Dyspnea, Hyperpnea, Apnea. By the term dyspnea in its 



widest sense we mean any noticeable increase in the force or rate of 



the respiratory movements. As said above, such a condition may 



be caused either by stimulation of sensory nerves, particularly 



*Cohnstein and Zuntz, "Arch. f. die gesammte Physiol.," 42, 342, 1888. 



