622 PHYSIOLOGY OF RESPIRATION. 



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 following interruption of the placental circulation. 

 During the intra-uterine period it is evident that the fetal blood is 

 aerated by exchange with the maternal blood sufficiently well not 

 to act as a stimulus to the fetal respiratory center. The fetus is, 

 physiologically 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 respiratory 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 

 the pain nerves, or by an increased venosity of the blood, that is, 

 by an increase in the C0 2 or by a marked decrease in the oxygen. 

 Changes of other kinds in the composition of the blood, some of 

 which are considered in the next chapter, may also stimulate the 

 respiratory center and cause dyspnea. The dyspneic movements 

 naturally show many degrees of intensity corresponding with the 

 strength of the stimulus, and sometimes the initial stages are desig- 

 nated as hyperpnea, while the term dyspnea is reserved for the more 



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



