1 2 2 THE ME CHAN ISM OF THE CIR C ULA TION. 



capillaries, and lost in the arterial system. In conditions of local vaso- 

 dilatation, where the peripheral resistance is greatly lessened, the pulse 

 may travel through the capillaries into the veins. The steady increase 

 of velocity and decrease in pressure are masked in the venous system by 

 other agencies, besides the systole of the heart, which affect the venous 

 circulation. The discussion of these will be found in the sections on the 

 influences of gravity and respiration on the circulation. 



Influence of Kespiration on the Pulmonary and 

 Systemic Circulation. . 



In his classical researches on blood pressure, Stephen Hales 1 records 

 the following observation : — ■" When the blood has subsided a little in 

 the tubes which were fixed to the arteries of these dogs, it would, as in 

 the horse's, rise on a sudden considerably on deep sighing, as also 

 on pressing the dogs' bellies hard with the hand, the blood would 

 immediately rise about six inches, and subside as much on taking off the 

 hand ; and it was the same on several repetitions." Early in this 

 century the inspiratory suction power of the thorax was urged by 

 Carson 2 and Barry, 3 as a mechanism for forwarding the progression 

 of the blood in the veins. Carson estimated the elasticity of the 

 expanded lung of the sheep as equal to the tension of a column of water 

 seven inches high. Barry observes that if one end of a bent tube be 

 introduced down the jugular vein, and the other end rest in a vessel 

 containing water, then the water within the tube can be seen to rise 

 during each inspiration, and sink during each expiration. Although by 

 this experiment Barry proved that a negative pressure is established 

 in the vena 1 cavre during inspiration, yet the importance of the suction 

 power of the thorax was not generally admitted. In opposition to 

 such a doctrine, Davies 4 declared that on compressing the vena 

 cava inferior in the abdomen, the upper portion of the vein is not 

 emptied by the aspiration of the thorax. On the other hand, on 

 obliterating the vena cava just below the auricles, after first squeezing 

 the blood within it onward into the heart, the vein immediately fills up 

 again from below. Thus, he argued, the driving force of the left 

 heart is by itself sufficient to return the blood to the right heart, and 

 complete the circulation. In experiments with the hsemadynamometer, 

 Poiseuille and Magendie 5 noticed an elevation of arterial pressure 

 during the period of expiration, and so soon as Ludwig, 6 by the 

 completion of the kymograph, succeeded in taking tracings of the 

 arterial pressure, he observed that variations in arterial tension 

 constantly occurred synchronously with the movements of respiration. 



Accepting the explanation of these undulations which at first sight 

 appeared obvious, Ludwig taught that the expiratory rise of intrathoracic 

 pressure aids the systolic output of the heart, propels the blood onward 

 into the arteries, and raises the arterial tension. The researches of 

 Donders, on the changes of intrathoracic pressure during the phases of 



1 Hales, "Statical Essays," London, 1733, vol. ii. p. 33. 



2 " Inquiry into the Causes of the Motion of the Blood," Liverpool, 1815. 



3 " Exper. Researches on Influence of Atmospheric Pressure upon Progression of Blood 

 in the Veins," London, 1826. 



4 Lancet, London, vol. xi. p. 606. 



5 Jrmm. de physiol. exper., Paris, tome i. p. 132. 



6 Arch. f. Anat., Physiol, u. wissensch. Med., 1847, S. 242. 



