THE BLOOD-PRESSURE IN THE ARTERIES. 167 



in this way influences the blood-pressure. The changes are greatest 

 in the arteries nearest the thorax. 



The respiratory variations in blood-pressure are in part dependent 

 upon changes in the nervous impulses sent out by the vasomotor center, 

 which coincide with the respiratory movements, and by virtue of which 

 the arteries contract and thus increase the arterial pressure (Traube- 

 Hering's pressure-variations). Fig. 65 III shows a respiratory curve 

 (heavy line) and a blood-pressure curve traced at the same time. This 

 figure shows that at the instant when expiration begins (at ex), the 

 blood-pressure curve rises along with the expiratory pressure, and, con- 

 versely, that both curves fall from the instant that inspiration begins 

 (at in) ; yet the blood-pressure curve begins to rise a little earlier (at c) 

 than expiration itself has begun, that is, during the last part of inspira- 

 tion. This is due to the contraction of the arteries, which begins a little 

 earlier in obedience to impulses sent out by the vasomotor center. The 

 effect of the arterial contraction is reinforced by the circumstance that 

 during the inspiratory stage the heart is more completely emptied on 

 account of the increased venous flow. The respiratory variations in 

 blood-pressure are observed also during artificial respiration ; if this be 

 suddenly interrupted (in curarized animals), the resulting irritation of 

 the medulla oblongata due to the dyspnea causes a considerable rise in 

 the blood-pressure. 



In accordance with the depth of the respirations and the corresponding pres- 

 sure-variations of the air within the thorax, great inequalities are observed" in the 

 respiratory fluctuations. This is evident. from the fact that in man during quiet 

 inspiration the diminution of pressure in the trachea is equivalent to only i mm. 

 of mercury, while during the deepest possible inspiration (with the respiratory 

 canal tightly closed) the diminution is 57 mm. Conversely, quiet expiration in 

 man is attended with an increase in the pressure in the trachea of only 2 or 3 

 mm., while vigorous contraction of the abdominal muscles causes an increase of 

 87 mm. of mercury. 



Kronecker and Heinricius attribute the variations to mechanical causes, 

 namely to the compression of the heart that accompanies respiration (because, 

 according to them, rhythmical injections of air into the pericardium, which com- 

 press the heart, also give rise to analogous variations in blood-pressure). Any 

 interference with the diastole of the heart lowers the blood-pressure; as soon, 

 therefore, as the lung has been distended during inspiration sufficiently to displace 

 the heart, diastole is interfered with and the tension in the aortic system is in 

 consequence lowered. As soon as the air can escape from the lungs and these 

 organs contract, a greater quantity of blood enters the heart, and the arterial 

 pressure rises. 



The movements of the pulse cause intermittent variations in the 

 mean arterial pressure, the so-called pulsatory pressure-variations. The 

 column of blood injected into the aortic system by the ventricle at each 

 systole, acting in conjunction with the positive wave, produces an in- 

 crease of pressure in the arterial system corresponding to this positive 

 wave. The increase in pressure finds corresponding expression in the 

 various elevations of the sphygmogram ; it also travels along the arteries 

 with the same velocity as the pulse-waves. 



In the larger arteries of the horse Volkmann found the pulsatory increase of 

 pressure to be T ^, and in the dog T V of the total pressure. Hurthle/with the aid 

 of his hemodynamometer, found that the pulsatory increase of pressure in the 

 rabbit was equal to almost one-third of the pressure during the interval between 

 pulse-beats. 



None of the pressure-recording instruments described shows the form of these 

 pressure-variations with sufficient accuracy; most of them merely record elevations 



