RESPIRATORY UNDULATIONS IN THE BLOOD-PRESSURE CURVE. 169 



dilatation of the abdominal blood-vessels, and consequent lowering of 

 the blood-pressure, is also called the depressor nerve. 



(/.) Respiratory Undulations. The arterial pressure also undergoes 

 regular variations or undulations owing to the respiratory movements. 

 These undulations are called respiratory undulations (Figs. 74 and 

 77). Stated broadly, during every strong inspiration the pressure 

 rises, and during expiration it falls ( 74). This is not quite correct 

 (see below). These undulations may be explained by the fact, that 

 with every expiration, the blood in the aorta is subjected to an increase 

 of pressure through the compressed air in the chest ; with every 

 inspiration, on the other hand, it is diminished owing to the diminu- 

 tion of the air in the lungs acting upon the aorta. Besides, the 

 inspiratory movements of the chest aspirate blood from the venae 

 cavse towards the heart, while expiration retards it, and thus influences 

 the blood-pressure. The undulations are most marked in the arteries 

 lying nearest to the heart. The respiratory undulations are due in part 

 to a stimulation or condition of excitement of the vaso-motor centre, 

 Avhich runs parallel with the respiratory movements. This stimulation 

 of the vaso-motor centre causes the arteries to contract, and thus the 

 blood-pressure is raised. The variations in the pressure which depend 

 upon a varying activity of the vaso-motor centre are known as the 

 curves of Traube and Hering (p. 171). In Fig. 77 are represented a 

 blood-pressure tracing and a curve of the movements of respiration 

 (thick line) taken simultaneously in a dog by C. Ludwig and Einbrodt. 

 The blood-pressure tracing was obtained from the carotid artery, while 



Fig. 77. 



Kyinographic blood-pressure tracing (upper, thin line), and respiration curve 

 (lower, thick line), taken simultaneously ex, expiration ; in, inspiration ; c, c, 

 heart-beats. The large curves in the blood-pressure tracing are due to 

 respiration (Ludwig and Einbrodt). 



the pressure within the thorax was measured by means of a manometer 

 placed in connection with one pleural cavity. In this curve, when 

 expiration begins (at ex), and as the expiratory-pressure rises, the blood- 

 pressure rises, while when inspiration begins (at in) both fall. The 

 blood-curve, however, begins to rise (at c) before expiration com- 



