74 CIRCULATION OF THE BLOOD IN THE VESSELS. 



arteries nearest the heart, and that it gradually diminishes toward the capil- 

 laries. The difference is very slight, almost inappreciable, except in vessels 

 of very small size ; but here the pressure is directly influenced by the dis- 

 charge of blood into the capillaries. The cause of this diminution of press- 

 ure in the smallest vessels is the proximity of the great outlet of the arteries, 

 the capillary system ; for, as will be seen farther on, the flow into the capilla- 

 ries has a constant tendency to diminish the pressure in the arteries. 



Influence of Respiration. It is easy to see in studying the arterial press- 

 ure, that there is a marked increase with expiration and a diminution with 

 inspiration. 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 least does not feel the 

 expulsive influence which follows with the act of expiration. 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 expi- 

 ration, in the jet from a divided artery. Under these conditions the ar- 

 terial pressure is at its maximum. In perfectly tranquil respiration, the 

 changes due to inspiration and expiration are slight, presenting a difference 

 of not more than half an inch or an inch (12*7 or 25'4 mm.) in the cardi- 

 ometer. When the respiratory movements are exaggerated, the oscillations 

 are very much more marked. 



Interruption of respiration is followed by a very great increase in the ar- 

 terial pressure. This is due, not to causes within the chest, but to obstruction 

 to the circulation in the capillaries. With an interruption of the respiratory 

 movements, the non-aerated blood passes into the arteries but can not flow 

 readily through the capillaries, and as a consequence, the arteries are abnor- 

 mally distended and the pressure is greatly increased. If respiration be per- 

 manently arrested, the arterial pressure becomes, after a time, diminished be- 

 low the normal standard, and is finally abolished on account of the stoppage 

 of the action of the heart. If respiration be resumed before the action of 

 the heart has become arrested, the pressure soon returns to its normal 

 standard. 



Influence of Muscular Action etc. Muscular effort considerably increases 

 the arterial pressure. This is due to two causes. In the first place, the 

 chest is generally compressed, and this favors the flow of blood into the great 

 vessels. In the second place, muscular exertion produces a certain degree of 

 obstruction to the discharge of blood from the arteries into the capillaries. 

 Experiments upon the inferior animals show a great increase in pressure in 

 the struggles which occur during severe operations. It has been shown that 

 stimulation of the sympathetic in the neck and of certain of the cerebro-spi- 

 nal nerves increases the arterial pressure, probably from an influence on the 



