232 A MANUAL OF PHYSIOLOGY 



have been emptied by suction and then permitted of themselves 

 to expand (inspiratory apnoea). The apnoea is not produced, 

 as some have thought, by the accumulation of an excess of oxygen 

 in the blood, for rapid and repeated inflation of the lungs with 

 hydrogen may cause the condition. Indeed, towards the end 

 of the apnoeic period the venous blood may be very distinctly 

 poorer in oxygen than normal venous blood. Apnoea is 

 easily caused in man by a period of deep and rapid breathing 

 and in other ways. The essential thing in this chemical or true 

 apnoea (apnoea vera) is the lowering of the partial pressure of 

 carbon dioxide in the alveolar air, and therefore in the arterial 

 blood and the respiratory centre. The carbon dioxide is washed 

 out of the body, so to say, by the excessive pulmonary ventilation. 



In addition to chemical apnoea, which is obtainable whether 

 the vagi are intact or no^ a so-called mechanical apnoea, or 

 apnoea vagi, exists that is to say, a stoppage of the respiration 

 due to an inhibitory effect produced through the vagi on the re- 

 spiratory centre when the vagus endings in the lungs are excited 

 mechanically by inflation. Some observers state that this vagus 

 apnoea does not outlast the inflation. Others believe that the 

 results of successive inflations can be ' summated ' in the 

 centre, giving rise to an apnoea which persists after stoppage 

 of the artificial respiration. That a ' memory ' of a prolonged 

 rhythmical inflation of the lungs can impress itself in some way 

 on the respiratory centre is shown by the curious phenomenon 

 that in resuscitation of the bulb after a period of anaemia the 

 natural respiration, when it returns, may have for a short time 

 exactly the same rhythm as the artificial respiration which has 

 just been stopped. 



That the blood when the gaseous exchange in the lungs is inter- 

 fered with produces dyspnoea by acting on some portion of the 

 brain may be shown in an interesting manner by establishing what 

 is called a cross-circulation in two rabbits or dogs. The vertebral 

 arteries and one carotid are tied in both animals ; the remaining 

 carotids are divided and connected crosswise by glass tubes, or, 

 what is better, as it avoids the risk of clotting, they are crossed 

 by suturing the cut ends, so that the brain of each is supplied 

 by blood from the other. When the respiration is artificially 

 hindered or stopped in one of the animals, it shows no dyspnoea ; 

 it is in the other, whose brain is being fed with improperly venti- 

 lated blood, that the respiratory movements become exaggerated. 

 The point of attack of the ' venous ' blood has been further 

 localized in the spinal bulb by the observation that when the 

 brain has been cut away above it, the cord severed below the 

 origin of the phrenics, and all other nerves connected with the 

 region between the two planes of section divided, any interference 



