CIRCULATION. 407 



Changes in the Beating Auricles. Except in small animals, the walls 

 of both the ventricles are so thick that the color of the two is the same and 

 is unchanging, namely, that of their muscular mass; but the walls of the 

 auricles are so thin that their color is affected by that of the blood within, so 

 that the right auricle looks bluish and dark and the left auricle red and 

 bright. During the brief systole of the auricles they are seen to become 

 smaller and paler as blood is expelled from them, while their serrated edges 

 and auricular appendages shrink rapidly away from the observer. The 

 changes of the auricular systole are seen to precede immediately the changes 

 of the systole of the ventricles and to succeed the repose of the whole heart. 

 During the relatively long diastole of the auricles these are seen to swell, 

 whether the ventricles are shrinking in systole or are swelling during the 

 first and greater part of their diastole. 



Changes in the Great Veins. In the venae cava3 and pulmonary veins a 

 pulse is visible, more plainly in the former than in the latter, which pulse has 

 the same rhythm as that of the heart's beat. The causes of this pulse are 

 complex and imperfectly understood. It depends in part upon the rhythmic 

 contraction of muscular fibres in the walls of the veins near the auricles, which 

 must heighten the flow into the latter, and which contraction the auricular 

 systole immediately follows. 1 This venous pulse will be mentioned again in 

 discussing the details of the events of the cycle (see p. 430). 



Changes in the Great Arteries. It is interesting to note that even in 

 so large an animal as the calf the pulse of the aorta or of the pulmonary 

 artery can hardly be appreciated by the eye, so far as the increase in girth of 

 either vessel is concerned. The expansion of the artery affects equally all 

 points in its circumference, and being thus distributed, is so slight in propor- 

 tion to the girth of the vessel that the profile of the latter scarcely seems to 

 change its place. The lengthening of the expanding artery can be more 

 readily seen. 



Effects of Opening the Chest. Such are the changes observed in the 

 heart and vessels when exposed in the opened chest of a mammal lying on 

 its back. The question at once arises, Can these changes be accepted as iden- 

 tical with those which occur in the unopened chest of a quadruped standing 

 upon its feet, or of a man standing erect? It will be most profitable to deal 

 at once with the case of the human subject. What are the possible, indeed 

 probable, differences between the changes in the heart in the unopened upright 

 chest and in the same when opened and supine ? 



When air is freely admitted to both pleural sacs, all those complex effects 

 upon the circulation are at once abolished which we have seen to be caused 

 by the elasticity of the lungs and the movements of respiration. The arti- 

 ficial respiration will have an effect upon the pulmonary transit of the blood 

 and so upon the circulation ; but the details of this effect are not the same as 

 those of natural respiration, and, for our present purpose, may be disregarded. 



1 T. Lauder Brunton and F. Fayrer : "Note on independent Pulsation of the Pulmonary 

 Veins and Vena Cava," Proceedings of the Royal Society, 1876, vol. xxv. p. 174. 



