THE INFLUENCE OF RESPIRATION. 



129 



raised above the pressure exerted on the extrathoracic vascular system, the 

 blood congests in the veins, and the heart fails to fill. 



In the case of the balloonist or the diver, both the mtra-pulmoiiary and 

 extrathoracic pressures fall or rise together, and thus the circulatory mechanism 

 remains unaffected. 



The frequency of the heart is increased by inspiration, and as this 

 change is brought >about by a lessening of the vagal tone, it fails to 

 appear when the vagi are cut. How the vagus centre is thus influenced 

 is not clear. Moderate artificial expansion of the lung increases the 

 frequency of the heart. Fredericq finds the change in frequency still 

 occurs synchronously with the respirations when the thorax is widely 

 open, the lungs collapsed, and the respiration ineffectual. 1 The vaso- 

 motor centre is also said to be influenced rhythmically and directly by 

 the respiration. To show this, Fredericq opened the thorax and belly 

 of an animal, divided both vagi and phrenic nerves, and stopped the 

 artificial respiration. The aortic pressure then rose with each expiratory 

 and fell with each inspiratory spasm. This experiment is not conclusive, 

 for although the suction action of the thorax and the compressing 

 action of the abdominal muscles were to a great extent abolished, yet 

 the general asphyxial spasms of the muscles still promoted the venous 

 flow and the diastolic filling of the heart. The three centres the re- 

 spiratory, the cardio-inhibitory, and the vasomotor lie closely bound 

 up together in the medulla oblongata, and it is therefore conceivable, 

 although not proved, that the other two may be directly influenced by 

 irradiation of the rhythmic discharge from the respiratory centre. The 

 exciting agent is certainly not the chemical nature of the blood, for the 

 alteration of frequency takes place when the thorax is opened and 

 the respiration is totally ineffectual. The inspiratory acceleration may 

 be due to local mechanical excitation of the heart, since, during vagal 

 inhibition, respiratory gasps or compressions of the abdomen excite 

 beats of the heart. 



From the preceding discussion the following conclusions can be 

 tabulated 2 as to the effect of respiration on the circulation : 



The total effect on the aortic tension in slow-breathing animals is maximal 

 at the commencement of expiration, minimal at the beginning of inspiration. 



1 Arch, de bioL, Gand, 1882, tome iii. p. 86. 



2 After Tigerstedt, " Phys. des Kreislaufes," S. 460. 



VOL. II. 9 



