TRANSACTIONS OF THE SECTIONS. 455 



serious injury to the individual, they also retain their vitality 

 long after their separation from the rest of the system. From 

 the same causes very young animals appear to suffer less pain 

 during experiment than those of mature age. 



After discussing the methods of experimenting, the author 

 proceeds to describe the opinions virhich have been held by 

 other persons on the subjects in question, and to compare them 

 with the conclusions to vi^hich his experiments have led. 



1st. It has been asserted by Bichat, and his celebrity has 

 induced many to adopt the opinion, that the ventricles possess 

 a power of active dilatation, by means of which, when their 

 systole has terminated, they are enabled to invite into their 

 cavities the blood from the neighbouring auricles. The author, 

 however, has ascertained by experiment that there is no such 

 dilating power in the ventricles, but that these muscles, when 

 their state of contraction has ceased, become perfectly soft and 

 flaccid, like all other muscles in their state of repose, and thus 

 readily admit the blood from their respective auricles, which 

 had become distended during the systole of the ventricles. The 

 feeling of resistance which was mistaken by Bichat for a dilating 

 power, and was supposed by him to accompany the diastole of 

 the ventricles, the author has ascertained to be caused by the 

 swelling of their muscular fibres during their systole. 



The auricles contract but little upon their contents in man 

 and in the higher classes of animals, the small quantity of blood 

 which the ventricles discharge at each contraction being com- 

 pensated by the frequency of their movement ; while in the 

 cold-blooded animals, in which the heart acts with less fre- 

 quency, the degree of expansion and contraction of both au- 

 ricle and ventricle is much greater than in the former classes, 

 and the quantity of blood sent through the heart at each move- 

 ment is much larger. 



2nd. The impulse of the heart against the side of the chest, 

 commonly called its beat, has been explained by different 

 writers in various ways. Mr. Hunter supposed it to have been 

 caused by the straightening of the curve of the aorta during 

 the systole of the ventricles, whereby the apex of the heart was 

 thrown forwards. Meckel refers it, in part, to the elongation 

 of the arterial tubes during the ventricular contraction, and 

 partly to the swollen state of the auricles at that time, by which 

 the ventricles are pushed forward against the side of the chest. 

 Harvey mentions an opinion held by some in his time, and which 

 has been lately revived, namely, that the beat is caused, not by 

 any active power in the ventricles, but by the muscular con- 

 traction of the auricles during their systole, by which the blood 



