408 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



What has been abolished is the continual suction, rhythmically increased in 

 inspiration, exerted by the lungs upon the heart and all the vessels within the 

 chest, which suction at all times favors the expansion and resists the con- 

 traction of the cavities of the heart and of the vessels. On the opening of 

 both pleural sacs the heart and vessels are exposed to the undiminished and 

 unvarying pressure of the atmosphere. Moreover, the heart has ceased to be 

 packed, as it were, between the pleurae and lungs to right and left, the spine, 

 the front of the chest-wall, and the diaphragm. From these considerations it 

 follows that the heart must be freer to change its form and position in the 

 opened than in the unopened chest; and that these changes must be more 

 modified by simple gravity in the former case than in the latter. Even in 

 the open chest we have studied these changes only in an animal lying on its 

 back. But if we turn the creature to either side, or place it upright in imi- 

 tation of the natural human posture, the ventricles of the exposed heart in 

 any case tend to assume, in systole, the same form, which has been com- 

 pared roughly to a right cone with a circular base. This is the form proper 

 to the hardened structure of branching and connected fibres of which the 

 contracting ventricles consist. But if the exposed ventricles be noted in dias- 

 tole, it will appear that their form depends very largely upon the effects of 

 gravity upon the exceedingly soft and yielding mass formed by their relaxed 

 fibres. We have seen them, in diastole, to flatten from breast to back, to 

 spread out from side to side, to gravitate toward the tail and to the left. If 

 the animal is laid on its side, they flatten from side to side, they spread 

 out from breast to back, and gravitate to the right or left, as the case 

 may be. 1 



Probable Changes in the Heart's Form and Position in the Unopened 

 Chest. It is fair to conjecture that the increase of the relaxed ventricles in 

 girth and in length which is seen in the open chest would not be greatly differ- 

 ent in the closed chest of a man in the upright posture. But it is probable 

 that the flattening of the exposed heart from breast to back, which is seen in 

 diastole, would not occur if the chest were closed. It is precisely in this direc- 

 tion that the flaccid heart exposed in the supine chest would be flattened un- 

 duly by its own weight, when deprived of many of its anatomical supports 

 and of the dilating influence of the lungs. The flattening from breast to back 

 must cause an exaggerated spreading out from side to side and hence an unduly 

 elliptical form of the base, inasmuch as, at the same time, the girth of the ven- 

 tricles is increasing as they enlarge in their diastole. Conversely, it is prob- 

 able, both a priori and from experimental evidence, that in the chest, when 

 closed and upright, the diminution in size of the contracting ventricles pro- 

 ceeds more symmetrically ; that their girth everywhere diminishes through a 

 diminution of the diameter from breast to back as well as of that from side to 



1 J. B. Haycraft: "The Movements of the Heart within the Chest-cavity, and the Cardio- 

 gram," The Journal of Physiology, vol. xii., Nos. 5 and 6, December, 1891, p. 448 ; J. B. Hay- 

 craft and D. R. Paterson : " The Changes in Shape and in Position of the Heart during the 

 Cardiac Cycle," The Journal of Physiology, vol. xix., Nos. 5 and 6, May, 1896, p. 496. 



