OF VITAL MOTION. 107 



which tissue had been compressed during the systole, 

 for no such structure is to be found : and therefore 

 we are led to suppose that the ventricular diastole 

 must be more nearly allied to dilatation than 

 relaxation. 



Again: the peculiar construction of the heart in 

 insects, and other articulate animals, is in favour of 

 the existence of a state of active dilatation in this 

 organ. In these creatures, the heart lies in the 

 general cavity of the body, and floats in the blood- 

 like liquid with which this cavity is filled. There are 

 no venous trunks, but the blood at once finds entrance 

 into the ventricle through simple valved slits. The 

 heart, moreover, is not a dense muscular viscus, as in 

 the higher animals, but it is a lax membranous tube, 

 little, if at all, superior to the condition of an ordi- 

 nary small vessel. Notwithstanding this peculiarity, 

 however, the beats follow the characteristic rhythm. 

 The action, in fact, is perfect with the conditions sim- 

 plified. Here, for example, the diastole cannot be a 

 state of mere relaxation originating in the operation 

 of a vis-a-tergo, for in a heart without veins, and 

 lodged, as it were, in an immense auricle where the 

 blood moves in no definite current, there can be no 

 such force. Here, moreover, the diastole cannot be 

 the consequence of the resilience of elastic tissue, for 

 the walls of the organ are lax and membranous. In 

 such a heart, indeed, there can be no passive cause of 



