INFLUENCE OF RESPIRATION. 77 



caliber, which is here reduced to capillary fineness. The latter tube is designed to give the 

 mean arterial pressure ; the constricted portion offering such an obstacle to the rise of 

 the mercury that the intermittent action of the heart is not felt, the mercury rising slowly 

 to a certain level, which is constant and varies only with the constant pressure in the 

 vessels. 



We have only an approximative idea of the average pressure in the arterial system in 

 the human subject, deduced from experiments on animals. It has already been stated to 

 be equal to about six feet of water or six inches of mercury. 



The most interesting questions connected with the subject of the arterial press- 

 ure are the comparative pressure in different parts of the arterial system, the conditions 

 which modify the arterial pressure, and its influence on the pulse. These points have all 

 been pretty fully investigated by experiments on animals and observations on systems of 

 elastic tubes arranged to represent the blood-vessels. 



Pressure in Different Parts of the Arterial System. The experiments of Hales, Poi- 

 seuille, Bernard, and others, seem to show that the constant arterial pressure does not 

 vary in arteries of different sizes. These physiologists have experimented particularly 

 on the carotid and crural, and have found the pressure in these two vessels about the 

 same. From their experiments they conclude that the force is equal in all parts of the 

 arterial system. Th experiments of Volkmann, however, have shown that this conclu- 

 sion has been too hasty. With the registering apparatus of Ludwig, he has taken the 

 pressure in the carotid and the metatarsal arteries and has always found a considerable 

 difference in favor of the former. In an experiment on a dog, he found the pressure equal 

 to one hundred and seventy-two millimeters in the carotid, and one hundred and sixty- 

 five in the metatarsal. In an experiment on a calf, the pressure was one hundred and 

 sixteen mm. in the carotid, and eighty-nine mm. in the metatarsal; and in a rabbit, 

 ninety-one mm. in the carotid, and eighty-six mm. in the crural. These experiments show 

 that the pressure is not absolutely the same in all parts of the arterial system, that it is 

 greatest in the arteries nearest the heart, and that it gradually diminishes as we near the 

 capillaries. The difference is very slight, almost inappreciable, until we come to vessels of 

 very small size ; but here the pressure is directly influenced by the discharge of blood into 

 the capillaries. The cause of this diminution of pressure in the smallest vessels is the prox- 

 imity of the great outlet of the arteries, the capillary system ; for, as we shall see farther 

 on, the flow into the capillaries has a constant tendency to diminish the pressure in the 

 arteries. It is obvious that this influence can only be felt in a very marked degree in the 

 vessels of smallest size. 



Influence of Respiration. It is easy to see, in studying the arterial pressure with any 

 of the instruments we have described, that there is a marked increase with expiration 

 and a diminution with inspiration. The fact that expiration will increase the force of the 

 jet of blood from a divided artery has long been observed and accords perfectly with the 

 above statement. 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 expelled 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 arterial pressure is at its maximum. In perfectly tranquil 

 respiration, the changes due to inspiration and expiration are slight, presenting a differ- 



