298 MANUAL OF PHYSIOLOGY. 



responding to the respiratory movements is not quite so simple as 

 it might appear to be at first sight, and it has often been misun- 

 derstood. Though many causes have been given, no single one 

 appears to explain adequately all the changes that may occur in 

 this phenomenon under varying circumstances. At first sight 

 the respiratory movements and consequent pressure changes 

 within the thorax would seem to give a simple mechanical ex- 

 planation. But if the change occurring in the iutrathoracic 

 pressure be examined carefully, it will 

 FIG. 137. be found not to correspond exactly with 



the so-called respiratory wave of the 

 pressure curve in the arterial system. 

 Owing to the lungs being very elastic 

 and constantly tending to shrink away 

 from the costal pleura, as may be seen 



Tracing of blood pressure when the thoracic Cavit - y is P e ed * nd 



taken with Tick's mano- the lun gs collapse, the pre>sure in the 



meter. pleural cavity is less than that of the 



atmosphere which distends the lungs, 



i.e., the pleural pressure is negative. All the viscera in the 

 thoracic cavity are habitually under the influence of the negative 

 pressure. Thus the elastic lungs exert a kind of traction on the 

 pericardium, and tend to cause a negative pressure within the 

 heart and great systemic vessels, both arteries and veins. The 

 influence is of course greater in the thin-walled veins, in which 

 the pressure is minimal, than in the thick-walled arteries, where 

 the pressure is so high that they feel but little the iutrathoracic 

 decrease. 



The amount of traction exercised on the pericardial contents 

 by the lungs varies with the respiratory movements, being slightly 

 increased during inspiration, and decreased during expiration. 

 The differences which are thus produced however during ordinary 

 respiration are very slight (probably 1 mm., mercury), when 

 compared with the mean negative pressure, which, while the 

 thorax is in an intermediate state of extension, is probably 

 about 10 mm., mercury. So slight a variation as 1 mm., mer- 

 cury, could not, by direct action on the aortic arch, cause the 



