THE CIRCULATION THROUGH SPECIAL ORGANS 431 



variations in their caliber in consequence of the respiratory movements 

 of the thorax. They are widened during normal inspiration and 

 compressed during expiration. This leads us to infer that the through 

 flow is greatest during the former phase, because the resistance is 

 least at this tune. But if the lungs are distended artificially through 

 the trachea, these conditions are reversed, because their inflation with 

 air produces a compression of their blood-vessels. The peripheral 

 resistance is increased during the inflation. Conversely, it may be con- 

 cluded that the deflation of these organs enables the vessels to acquire 

 their previous caliber. This change is associated with a diminution 

 in the peripheral resistance. 1 As has previously been noted, these 

 rhythmic variations in the conditions inside the thorax play an 

 important part in the production of the respiratory oscillations in 

 blood pressure. Attention should also be called at this time to the 

 fact that the vascularity of the lungs is subject to the conditions pre- 

 vailing in the heart. Any momentary excess in the venous influx 

 must, of course, be accommodated by the distended pulmonary chan- 

 nels until the heart is again capable of propelling it. A hyperemia of a 

 more permanent kind, however, must result whenever the left ventricle 

 is unable to relieve the lungs of a normal quantity of blood. A con- 

 dition of the kind must arise during stenosis or regurgitation of the 

 mitral or aortic valves. The lesser circuit, therefore, is capable of 

 acting as a reservoir, the purpose of which is to equalize the flow 

 through the heart. 



The measurements of the pressure and flow in the pulmonary 

 artery meet with serious difficulties, because the insertion of a cannula 

 in this blood-vessel or in any of its branches necessitates in many 

 animals the opening of the pleurae and a temporary blocking of the 

 pulmonary circulation. In rabbits, however, it is possible to gain 

 free access to the heart by simply dividing the sternum in the median 

 line. 2 As the pleural sacs do not quite reach to this line, they need 

 not be opened and artificial respiration need not be resorted to. Beut- 

 ner 3 has given the following values which have not been materially 

 changed in more recent years: 



Dog 28-31 mm. Hg 



Cat 15-19 mm. Hg 



Rabbit 9-17 mm. Hg 



These figures harmonize completely with the fact that the right ven- 

 tricle develops much lower pressures than the left, without, however, 

 causing the usual systolic-diastolic differences to disappear. But as 

 the latter show oscillations of only about 15 mm. Hg, as against 30- 

 40 mm. Hg in the systemic circuit, their range is rather limited. In 



1 Tigerstedt, Ergebnisse der Physiol., ii, 2, 1903; also see: Burton-Opitz, Am. 

 Jour, of Physiol;, xxxvi, 1914, 64. 



2 Knoll, Sitzungsber., Ak., Wien, xcvii, 207, 1888. 



3 Zeitschr. fur rat. Med., N. F., ii, Ser., 1882; also see: Bradford and Dean, 

 Proc. Royal Soc., London, 1889, and Schafer, Quart. Jour, of Exp. Physiol., xii, 

 1919, 133. 



