THE RESPIRATORY MECHANISM. 395 



rises, air and blood both tend to be expelled from the cavity. 

 The aorta thus regains what it lost during inspiration; the 

 pressure on it is increased and it empties itself faster into its 

 abdominal portion. The semilunar valves having prevented 

 any regurgitation into the heart, there is neither gain nor 

 loss so far as it is, concerned. With the systemic intra-tho- 

 racic veins, however, this is not the case; the extra blood en- 

 tering them has already in great part gone on beyond the 

 tricuspid valve, and cannot flow back during expiration ; 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 cavae themselves contain no valves, 

 the more distant veins which open into them do. Conse- 

 quently, whatever extra blood has, to use the common phrase, 

 been '* sucked" into the intra-thoracic venae cavae in inspira- 

 tion 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 au- 

 ricle during the diastole of the latter. How much the alter- 

 nating respiratory movements assist the venous flow is shown 

 by the dilation of the veins of the head and neck which oc- 

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

 for the face, from distention of the veins and stagnation of 

 the capillary flow, which occurs during a prolonged fit of 

 coughing, which is a series of expiratory efforts without any 

 inspirations. 



On the whole the influence of the respiratory movements 

 on the blood-flow is such as to favor it in inspiration and to 

 impede it during expiration. This influence very often shows 

 itself on tracings of arterial pressure taken as described in 

 Chap. XVIII. Such tracings usually show in addition to the 

 pulse waves, slower and greater rises and falls of pressure 

 which have the same rhythm as the respiration. In general, 

 the rise of pressure in these respiratory waves of blood-pres- 

 sure is synchronous with inspiration and the fall with expira-- 

 tion, but not exactly. The changes manifest themselves on 

 the blood-pressure curve a little later than the commencement 

 of the thoracic movement which leads to them; the rise be- 



