CHAP. IL] RESPIRATION. 647 



the blood-pressure, there are causes at work which in each case 

 delay the effect. 



Extended observations however shew that such a relation as 

 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 difficulty. 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 general effect varies according as 

 one or other influence is predominant. These several actions are 

 sufficiently interesting and important to deserve discussion. 



382. 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, as we have seen ( 324), the air in the pulmonary alveoli 

 is rarefied and brought to a lower pressure than that of the atmo- 

 sphere outside the chest ; and the difference of pressure thus set 

 up leads to an inrush of inspired air until an equilibrium 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 dis- 

 tension is effected by means of the pressure which causes the 

 inspired air to enter. Part of the atmospheric pressure in fact 

 which causes the entrance of the air into the lung is spent in over- 

 coming the elasticity of the pulmonary 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 elas- 



