706 PHYSIOLOGY OF RESPIRATION. 



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, and we may 

 assume that this condition is due, in part at least, to the absence 

 of somatic and cutaneous stimuli in the protected fetus. 



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 CO 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 

 designated as hyperpnea, while the term dyspnea is reserved for 

 the more labored breathing in which the expirations are active 

 and forced. When dyspnea is produced by withholding air 



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



