EFFECT OF RESPIRA TION ON BLOOD-FLO W. 369 



Tented any regurgitation into the heart, there is neither 

 gam noi loss so far as it is concerned. With the systemic 

 intrathoracic veins,, however, this is not the case; the extra 

 blood entering them has already in great part gone on be- 

 yond the tricuspid valve, and cannot flow back during ex- 

 piration; and the pressure in the auricle being constantly 

 kept low by its emptying into the ventricle, the increased 

 pressure on the venae cavae tends rather to send blood 

 on into the heart, than back into the extra-thoracic veins. 

 Moreover, whatever blood tends to take the latter course 

 cannot do it effectually since, although the venae cavse 

 themselves contain no valves, the more distant veins which 

 open into them do. Consequently, whatever extra blood 

 has, to use the common phrase, been "sucked" into the 

 intra-thoracic venae cavae in inspiration and has not been 

 sent already on into the right ventricle before expiration 

 -occurs, is, on account of the venous valves, imprisoned in 

 the cavae under an increased pressure during expiration; 

 ;and this tends to make it flow faster into the auricle during 

 the diastole of the latter. How much the alternating res- 

 piratory movements assist the venous flow is shown by the 

 dilation of the veins of the head and neck which occurs 

 when a person is holding his breath; and the blackness for 

 the face, from distension of the veins and stagnation of the 

 capillary flow, which occurs during a prolonged fit of cough- 

 ing, which is a series of expiratory efforts without any in- 

 spirations. 



In still another way the aspiration of the thorax assists 

 the heart. The heart and lungs are bothdistensible, though 

 in different degrees, and each is stretched in the chest 

 somewhat beyond its natural size; the one by the atmos- 

 pheric pressure directly, the other by that pressure in- 

 directly exerted through the blood exposed to it in the 

 extra-thoracic veins. Supposing, therefore, the heart sud- 

 denly to shrink it would leave more space in the chest to be 

 filled by the lungs; these must accordingly, at each cardiac 

 systole expand a little to fill the extra room, just as they do 

 Tvhen the space around them is otherwise enlarged during 



