394 THE HUMAN BODY. 



which they there accumulate compensates for the less exter- 

 nal pressure to which those vessels are exposed. An equilib- 

 rium would thus very soon be brought about were it not for 

 the respiratory movements, in consequence of which the 

 intra-thoracic pressure is alternately increased and dimin- 

 ished, and the thorax comes to act as a sort of suction-pump 

 on the contents of the vessels of the Body outside it; thus 

 the respiratory movements influence the circulation of the 

 blood and the flow of the lymph. 



Influence of the Respiratory Movements upon the Cir- 

 culation. Suppose the chest in a condition of normal expira- 

 tion and the external pressure on the blood in the blood-ves- 

 sels within it and in the heart, to have come, in the manner 

 pointed out in the last paragraph, into equilibrium with the 

 atmospheric pressure exerted on the blood-vessels of the neck 

 and abdomen. If an inspiration now occurs, the chest cavity 

 being enlarged the pressure on all of its contents will be di- 

 minished. In consequence, air enters the lungs from the 

 windpipe, and blood enters the venae cavae and the right au- 

 ricle of the heart. Thus not only the lungs, but the right 

 side of the heart, and the intra-thoracic portions of the sys- 

 temic veins leading to it, are expanded during an inspiration; 

 but the lungs being much the most distensible take far the 

 greatest part in filling up the increased space. The left side 

 of the heart is not much influenced as it is filled from the 

 pulmonary veins; and the whole vessels of the lesser circula- 

 tion lying within the chest, and being all affected in the 

 same way at the same time, the blood-flow in them is not di- 

 rectly influenced by the aspiration of the thorax. Distention 

 of the lungs seems, however, to diminish the capacity of their 

 vessels, and so to a certain extent the flow is influenced; as 

 the lungs expand blood is forced out of their vessels into the 

 left auricle, and when they again contract their vessels fill 

 up from the right ventricle. The pressure on the thoracic 

 aorta being diminished in inspiration, blood tends to flow back 

 into it from the abdominal portion of the vessel, but cannot 

 enter the heart on account of the semilunar valves; and the 

 back-flow does not in any case equal the onflow due to the 

 beat of the heart; so what happens in the aorta is but a 

 slight slowing of the current. The general result of all this 

 is that the circulation is considerably assisted. When the 

 next expiration occurs, and the pressure in the thorax again 



