RESPIRATION 287 



an extraordinary increase in frequency and diminution in depth 

 (to half or even a fourth) of respiration. At the same time the 

 rate of circulation (measured by a very perfect blood-gas method 

 described in the same journal by Barcroft, Boycott, Dunn and 

 Peters) was not diminished, nor was the venous blood pressure 

 raised, or the arterial pressure disturbed : nor was there appreci- 

 able deficiency of oxygen or excess of CO 2 in the arterial blood. 

 But when the vagi were cut the respirations slowed down and 

 became normally deep at once. It appears, therefore, that the 

 Hering-Breuer reflex (Chapter III) was enormously exaggerated 

 as a result of the disturbed pulmonary circulation. Just at first the 

 breathing was stopped, which suggests that the respiratory move- 

 ments were jammed completely by the exaggerated reflex. These 

 experiments throw a quite new light on the intense and exhausting 

 dyspnoea caused by pulmonary embolism, and also in cardiac cases 

 where there is rapid breathing without other cause. How the vagus 

 nerve endings are excited is not yet clear. The discovery of a drug 

 capable of controlling their action would evidently be an important 

 advance in therapeutics. 



In defective circulation owing to loss of blood the primary 

 cause of breakdown appears to be that, in spite of contraction of 

 arterioles and venules owing to pressor reaction of the vasomotor 

 center, there is not sufficient blood to fill the large veins and ade- 

 quately supply the right side of the heart. As a consequence the 

 arterial blood pressure falls and the circulation slows down, with 

 consequent anoxaemia acting most seriously on the brain, and 

 affecting the breathing in the manner already explained in con- 

 nection with valvular affections where compensation is imperfect. 

 The natural remedy for this condition would appear at first sight 

 to be a pressor excitation of the vasomotor center, just as the 

 natural remedy for arterial anoxaemia due, say, to low atmos- 

 pheric pressure, appears at first sight to be increased breathing 

 and increased circulation rate. But just as the increased breathing 

 and circulation rate in arterial anoxaemia is to a large extent pre- 

 vented by the counter-balancing effect of the alkalosis thereby 

 produced, so also is the full pressor response to anoxaemia due to 

 fall in blood pressure. The breathing is already stimulated by the 

 diminished blood circulation in the brain, so that the arterial 

 blood is so alkaline as to quiet down the vasomotor center, in spite 

 of the anoxaemia. Benefit may be expected from the administra- 

 tion of CO 2 or even of acids ; but the main need is for increase in 

 the volume of the blood. This increase comes naturally, provided 



