xiii KESPIEATOEY EHYTHM 477 



the usual venous colour, while the left auricle is at first of a bright 

 arterial scarlet, and then grows darker during the course of the 

 apnoea, a process which usually ceases only when the colour of the 

 left auricle has become much darker than the normal. This shows 

 that the mechanical action of artificial respiration with the bellows 

 causes by means of the vagi a diminution of excitability in the 

 respiratory centres. In fact a highly venous blood, which under 

 normal conditions would determine a rise of excitability in the 

 rhythmical activity of the centres, is unable, after vigorous artificial 

 respiration, to interrupt the apnoea. 



From these facts as a whole we may conclude that the apnoea 

 obtained with artificial respiration is the result of a certain ratio 

 between the venosity of the blood and the excitability of the 

 respiratory centres. Since it is thus possible to obtain apnoea even 

 when the venosity of the blood, far from being reduced, has 

 exceeded the normal limits : since, on the other hand, it does not 

 appear after section of the vagi, i.e., when the moderating influence 

 exerted by these nerves upon the excitability of the bulbar centres 

 when mechanically stimulated, is eliminated, it follows that the 

 said apnoea must depend principally upon the reduced excitability 

 of the respiratory centres. 



Miescher proposes to give the name of true apnoea to that which 

 is determined by the diminished venosity of the blood, and spurious 

 apnoea to that which depends on the diminished excitability of the 

 respiratory centres. Before accepting" this distinction, we must 

 inquire whether there is any true apnoea in Miescher's sense, and 

 whether it is possible to produce it artificially in man or other 

 animals. 



XII. Let us in the first place consider the phenomenon of 

 foetal apnoea. We know that under normal conditions, so long as it 

 is contained within the uterus, the foetus performs no respiratory 

 movements ; it is apnoeic. The placental circulation provides for 

 the exchange of gases necessary to the internal respiration of the 

 foetal tissues. The umbilical arteries conduct the blood that has 

 become venous in these tissues to the placenta, and the umbilical 

 veins reconduct the blood which has become arterial by gas 

 exchanges with the maternal blood, to the foetus. The trans- 

 formation of arterial into venous blood is, however, but little 

 accentuated in the foetus. According to Zweifel and Zuntz, the 

 colour of the blood in the umbilical arteries differs little from that 

 of the umbilical veins, showing that the foetus consumes little 

 oxygen, and gives off little carbonic acid. Enclosed within the 

 maternal womb, immersed in a tepid bath, it has no need to 

 provide for its own calorification ; the muscles and digestive 

 glands are in almost complete repose ; the heart is the only 

 foetal organ that functions with any activity, and consumes a 

 certain amount of energy (Pfliiger). At the seventh month the 



