268 CIRCULATION. 



In tranquil respiration, the influence upon the flow of 

 blood is due simply to the mechanical action of the thorax. 

 With every inspiration the air-cells are enlarged, as well as 

 the blood-vessels of the lungs ; the air rushes in through the 

 trachea, and the movement of the blood in the veins near the 

 chest is accelerated. At the same time the blood in the arteries 

 is somewhat retarded in its flow from the thorax, or at ]east 

 does not feel the expulsive influence which follows with the act 

 of expiration. The mean of the arterial pressure at that time 

 is at its minimum. With the expiratory act, the air is ex- 

 pelled by compression of the lungs, the flow of blood into the 

 thorax by the veins is retarded to a certain extent, while the 

 flow of blood into the arteries is favored. This is strikingly 

 exhibited in the augmented force, with expiration, in the jet 

 from a divided artery. Under these circumstances the arte- 

 rial pressure is at its maximum. 



In perfectly tranquil respiration, the changes due to in- 

 spiration and expiration are very slight, marked by a differ- 

 ence of not more than half an inch to an inch in the car- 

 diometer. When the respiratory movements are exaggerated, 

 the oscillations are very much more marked. 



Interruption of respiration is followed by a very great in- 

 crease in the arterial pressure. This is due, not to causes 

 within the chest, but to obstruction to the circulation in the 

 capillaries. We are already aware of the influence which 

 the flow of blood into the capillaries is constantly exerting 

 upon the arterial pressure. This tendency to diminish the 

 quantity of blood in the arteries, and consequently the 

 pressure, is constantly counteracted by the blood sent into 

 the arteries by the contractions of the heart. In interruption 

 of the respiratory function, the non-aerated blood passes into 

 the arteries, but refuses to pass through the capillaries ; and 

 as a consequence, the arteries are abnormally distended, and 

 the arterial pressure is enormously increased. If respiration 

 be permanently arrested, the arterial pressure becomes, after 

 a time, diminished below the normal standard, and ultimate- 



