452 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



diastolic distention of the heart and dilatation of the intrathoracic vessels ; yet, 

 notwithstanding that this powerful aspiratory action encourages the flow of an 

 extraordinarily large amount of blood into the thoracic- vessels, the heart-beats 

 may be very small, because intrathoracic negative pressure is so great that the 

 thin-walled auricles meet with great resistance while contracting; in conse- 

 quence, then, of this forced inspiratory effort little blood is driven through the 

 lungs to the left auricle and by the left ventricle into the general circulation, 

 ff we make the greatest possible expiratory effort, and maintain the expira- 

 tory phase with air-passages open, the heart-beats arc small, owing to the 

 small amount of blood which How- through the venae cavse to the right auri- 

 cle, and to the resistance offered by the compressed intrapulmonary vessels. 



If, after a most powerful expiration, we close the mouth and nostrils and 

 make a powerful inspiratory effort, the aspiratory effect of inspiration on the 

 heart and the blood-vessels is manifest to its utmost degree: the heart and the 

 vessels tend to undergo great dilatation, the blood-flow to the right auricle and 

 ventricle is increased, the intrapulmonary vessels and the heart become en- 

 gorged, and, owing to the powerful traction of the negative pressure upon the 

 heart, especially upon the right auricle, very little blood is forced through the 

 lungs to the left auricle and ventricle and subsequently into the general circu- 

 lation, thus causing a fall of blood-pressure; indeed, the heart-sounds and the 

 pulse may disappear. If now we make the most forcible inspiratory effort, 

 close the glottis, and make a powerful expiratory effort, not only is the air in 

 the lungs subjected to high positive pressure, but the heart and the great 

 vessels partake in the pressure-effects, the blood being forced from the pul- 

 monic circulation into the left auricle, thence by the ventricle into the aorta, 

 with the result of a temporary rise of blood-pressure. The pressure upon the 

 intrathoracic veins is so great that the flow of blood into the chest is almost 

 shut off, hence the veins outside the thorax become very much distended, as 

 seen in the superficial veins of the neck, and the heart is pressed upon to such 

 an extent that, together with the lessened supply of blood, the heart-sounds 

 and the radial pulse may disappear and the blood-pressure falls. 



The respiration into or from a spirometer (p. 427) containing rarefied or 

 compressed air modifies the blood-pressure curves. Inspiration of rarefied air 

 causes a greater rise of blood-pressure than when respiration occurs at normal 

 pressure, while during expiration, although the blood-pressure falls, it may 

 remain somewhat above the normal. The increase of pressure is due to the 

 aspiratory effort required to draw the air into the lungs, which effort also makes 

 itself felt to a more marked degree upon the heart and the intrathoracic and 

 intrapulmonary vessels, thus increasing the blood-flow through the pulmonary 

 circulation. During expiration air is aspirated from the lungs into the spi- 

 rometer, tending to dilate the intrathoracic and intrapulmonary vessels and the 

 heart and thus to aid the pulmonary circulation. After a time, however, there 

 is a fall of blood-pressure on account both of the engorgement of the thoracic 

 vessels and the accompanying depletion of the general circulation, and of the 

 distention of the heart and interference with its contractions. 



In.-piration of compressed air lessens the extent of, and may prevent, the 



