INFLUENCE OF RESPIRATORY PRESSURE ON THE HEART. 123 



lungs are more fully expanded (inspiration), and become less effective 

 in proportion as the lungs have already been contracted (expiration). 

 From this it follows : 



1. That in the most extreme expiratory position of the thorax, with 

 the greatest possible contraction of the pulmonary tissue, when, there- 

 fore, what is left of the effective elastic traction of the lungs is exceedingly 

 slight, but little blood enters the cavities of the heart; the heart during 

 diastole is small and contains but little blood. Accordingly, the systolic 

 contractions will be small, that is, a small pulse results. 



2. In the most extreme inspiratory position, when the elastic lungs 

 are distended to their utmost, the force of the elastic traction of the 

 lungs is, naturally, greatest, being in fact equivalent to 30 millimeters 

 of mercury. The effect of this traction may be great enough to counter- 

 act the contractions of the thin-walled auricles and auricular appendages 

 and prevent these structures from emptying their contents completely 

 into the ventricles. In cases of cardiac weakness it would even appear 

 as if the ventricular activity were impaired by the strong elastic pulmo- 

 nary traction, as the diminution in the strength of the heart-sounds that is 

 sometimes observed attests. The heart, therefore, is greatly distended 

 in diastole and filled with blood ; nevertheless the resulting pulse-waves 

 may be small in consequence of the limitation of auricular activity. 

 Thus, Bonders often found the pulse smaller and slower. 



3. When the thorax is in the position of moderate rest, a condition 

 in which the elastic traction of the lungs is of moderate strength only, 

 namely, 7.5 millimeters of mercury, the conditions for the action of the 

 heart are most favorable. On the one hand, diastolic distention of the 

 cavities of the heart is adequate, and, on the other hand, their complete 

 evacuation during systole is not impeded. 



A much greater influence on the action of the heart is exerted 

 by the increase or diminution in the intrathoracic pressure produced 

 voluntarily by muscular action. 



1. If the thorax is first brought into the position of deepest inspira- 

 tion, then the glottis is closed, and now the space within the chest is 

 greatly reduced with the aid of the expiratory muscles; the cavities of 

 the heart may be so greatly compressed as to cause momentary sus- 

 pension of the movement of the blood within them. In this position 

 the elastic traction is greatly diminished, and in addition the pulmonary 

 air, which is under high tension, exerts pressure on the heart and the 

 intrathoracic vessels. As no venous blood can enter the thoracic 

 cavity from without, the visible veins become enlarged, the blood is 

 driven more rapidly into the left heart, and the latter empties itself 

 into the circulation as quickly as possible. The lungs are, as a result, 

 anemic and the cavities of the heart empty. Therefore, there is plethora 

 in the greater circulation, associated with anemia in the lesser and in the 

 heart. The heart-sounds cease, the pulse disappears. 



2. If, conversely, the glottis is closed, while the thorax is in the 

 position of most extreme expiration, and the thoracic cavity is now for- 

 cibly dilated in inspiration, the heart is strongly dilated; for the cavities 

 of the heart are distended not only by the elastic traction of the lungs, 

 but also on account of the extreme rarefaction of the pulmonary air. 

 The contents of the veins are poured copiously into the right heart, 

 and in proportion as the right auricle and the ventricle are capable of 



