CHAP, ii.] RESPIRATION. 499 



that shewn in the figure though frequent is not constant. In 

 fact the effects of the respiratory movements on blood-pressure 

 are found to vary very widely according as the respiration is 

 quick or slow, easy and shallow, or laboured and deep, and 

 especially as the air enters into the chest readily or with diffi- 

 culty. Moreover, respiratory undulations of blood-pressure are 

 seen not only with natural but also with artificial respiration ; 

 in the latter the mechanical conditions are to a large extent the 

 reverse of those of the former, and might fairly be expected to 

 affect the circulation in a different way. The causation of these 

 respiratory undulations is in fact complex. The respiratory act 

 affects the vascular system in several different ways, and the 



feneral effect varies according as one or other influence is pre- 

 ominant. These several actions are sufficiently interesting 

 and important to deserve discussion. 



311. The heart and great blood vessels are, like the lungs, 

 placed in the air-tight thoracic cavity, and are subject like the 

 lungs to the pumping action of the respiratory movements. 

 Were there no lungs present in the chest, the whole force of the 

 expansion of the thorax in inspiration would be directed to 

 drawing blood from the extra-thoracic vessels towards the heart, , 

 and conversely in expiration the effect of the return of the thorax 

 to its previous dimensions would be to drive the blood thus 

 drawn in back again from the heart towards the extra-thoracic 

 vessels. And, even in the presence of the lungs, some of this 

 effect is still felt. The main purpose and the main result of the 

 expansion of the chest in inspiration is of course to draw air into 

 the lungs ; by that expansion the air in the pulmonary alveoli 

 is rarified and brought to a lower pressure than that of the 

 atmosphere outside the chest ; and the difference of pressure 

 thus set up leads to an inrush of inspired air until an equilib- 

 rium of pressure is established between the air in the lungs and 

 that outside the chest. Before however the inspired air can 

 fill a pulmonary alveolus the elastic walls of the alveolus have 

 to be distended, and that distension is effected by means of the 

 pressure which causes the inspired air to enter. Part of the 

 atmospheric pressure in fact w r hich causes the entrance of the air 

 into the lung is spent in overcoming the elasticity of the pul- 

 monary passages and cells. So that while by the inrush of 

 inspired air the difference of pressure between the air inside the 

 pulmonary alveoli and that outside the chest, brought about by 

 the thoracic expansion, is completely neutralized, the difference 

 between the pressure to which the parts lying within the thorax 

 but outside the lungs are exposed and that outside the chest is 

 not so completely neutralized. The pressure on these parts 

 always falls short of the pressure of the atmosphere by the 

 amount of pressure necessary to counterbalance the elasticity -of 

 the pulmonary passages and alveoli. Consequently, any struct- 



