149 



owing to the increase of the pressure the dicrotic wave must be less 

 during expiration; (4.) for the same reason the elastic elevations 

 are more distinct and occur higher in the curve near its apex. The 

 frequency of the pulse is slightly greater during expiration than during 

 inspiration. 



(b.) This purely mechanical effect of the respiratory movements is 

 modified by the simultaneous stimulation of the vasomotor centre 

 which accompanies these movements. At the beginning of inspiration 

 the blood-pressure in the arteries is lowest, but it begins to rise during 

 inspiration, and increases until the end of the inspiratory act, reaching 

 its maximum at the beginning of expiration. During the remainder 

 of the expiration the blood-pressure falls until it reaches its lowest 

 level again at the beginning of inspiration (compare 85, /), the pulse- 

 curves are similarly modified, and exhibit the signs of greater or less 

 tension of the arteries corresponding to the phases of the respiratory 

 movements (Klemensiewicz, Knoll, Schreiber, Lb'wit). [There is, as it 

 were, a displacement of the blood-pressure curve relative to the respira- 

 tory curve.] 



Forced Respiration. With regard to the effect produced on the 

 pulse-curve by a powerful expiration and a forced inspiration, observers 

 are by no means agreed. 



Valsalva's Experiment. Strong expiratory pressure is best produced 

 by closing the mouth and nose, and then making a great expiratory 

 effort ; at first there is increase of the blood-pressure and the formation 

 of pulse-waves resembling those which occur in ordinary expiration, 

 the dicrotic wave being less developed ; but, when the forced pressure 

 is long continued, the pulse-curves have all the signs of diminished 

 tension (Riegel, Frank, and Sommerbrodt). This effect is due to the 

 action of the vasomotor centre, which is affected reflexly from the 

 pulmonary nerves. We must assume that forced expiration, such 

 as occurs in Valsalva's experiment, acts by depressing the activity of 

 the vasomotor centre (compare Vol. ii.) Coughing, singing, and declaim- 

 ing, act like Valsalva's experiment, while the frequency of the pulse 

 is increased at the same time (Sommerbrodt). After the cessation 

 of Valsalva's experiment, the blood-pressure rises above the normal state 

 (Sommerbrodt), almost as much as it fell below it; the normal con- 

 dition being restored within a few minutes (Lenzmann). 



Muller's Experiment. When the thorax is in the expiratory phase, 

 close the mouth and nose, and take a deep inspiration so as forcibly to 

 expand the chest ( 60). At first the pulse- curves have the char- 

 acteristic signs of diminished tension, viz., a higher and more distinct 

 dicrotic wave; then the tension can, by nervous influences, be in- 

 creased, just as in Fig. 64, where C and R are tracings taken from the 



