150 THE MECHANISM OF THE CIRCULATION. 



the circulatory pathway through the lungs is open during vigorous artificial 

 respiration. 



In confirmation of this view, Landgraf' s observations on the rabbit may be 

 quoted. 1 He split the sternum in the middle line without opening the pleural 

 cavities or interfering with natural respiration. In this condition he found 

 that ligature of one pulmonary artery lowered the aortic pressure, markedly 

 lessened the filling of the left heart and dilated the right heart. The experi- 

 ments of Lichtheim, and also those of Landgraf, prove how markedly differ- 

 ent the pulmonary circulation is in the states of artificial and natural breathing. 



Apart from the effect of respiration, the pulmonary circulation may 

 be influenced passively by the following agencies : 



1. By changes in the rate and force of the heart-beat, the circulatory 

 system otherwise remaining constant. 



2. By congestion in the left auricle, a " back effect," consequent on 

 increased resistance to the aortic outflow. 



3. By a fall of pressure in the left auricle, due to diminished resist- 

 ance in the aortic outflow. 



4. By a " forward effect," or increased input into the right heart, 

 such as is produced by the sudden constriction of a large area of arteries 

 emptying the blood into the veins, or by compression of the abdomen. 



5. By a diminished input into the right heart, such as is produced by 

 loss of vascular tone. 



6. By the influence of gravity and the effect of position, producing 

 an increased or diminished input into the right heart. 



It is important to consider in further detail how, by a " back effect/ 

 a rise of aortic pressure may congest the pulmonary circulation. 



The left ventricle has to vary its power of contraction in accordance 

 with alterations of arterial pressure which are far greater than ever fall to 

 the lot of the right ventricle. The left ventricle is thus found to become 

 exhausted, while the right still continues unaffected. Thus on inducing 

 asphyxia, after dividing the vagi, the left heart begins, when the aortic 

 pressure reaches its maximum, to give weak and irregular beats, while 

 the right heart continues for a time to contract strongly and regularly. 



On stimulating the cervical spinal cord, Waller 2 found that the 

 left ventricle could not empty itself against the high aortic pressure 

 consequent on the general condition of vase-constriction. The left 

 auricle thus became congested, and even ceased to show any signs of 

 contraction. Meanwhile the right heart continued to beat normally, 

 and while the pressure in the left auricle rose to 25 or 30 mm. Hg, 

 the pressure in the right auricle was only 5 mm. Hg. The lungs act 

 to the left heart as a blood reservoir, as the liver and large veins 

 stand in the same relation to the right heart. Thus the " back effect " 

 on the right heart is small during a short period of increased resistance 

 to the aortic outflow. By the combined effect of stimulation of the 

 spinal cord, ligature of the aorta, and compression of the abdomen, the 

 pressure in the right auricle has been raised to 22 mm. Hg. In this 

 case a " back effect " is combined with a " forward effect," and the input 

 into the right heart is increased while the output is resisted. Finally, when 

 the output of the right heart also becomes incomplete, and the reservoirs 

 of the lungs are filled, the blood congests in the liver and large veins. 



Bradford and Dean observed that compression of the thoracic aorta, 



1 Ztsehr.f. klin. Med., Berlin, 1892, Bd. xx. S. 181. 



2 ArcJi.f. PhysioL, Leipzig, 1878, S. 525. 



