CHAP, iv.] THE VASCULAR MECHANISM. 137 



longitudinal diameter, but that the apex remains stationary on 

 account of the compensating downward thrust of the whole 

 ventricle. It has been urged however that this method is untrust- 

 worthy, and that similar movements of needles thus placed might 

 be produced by the twisting of the heart on its long axis, com- 

 bined with an approximation of the heart to the chest-wall. And 

 different conclusions have been arrived at by taking plaster of 

 Paris models on the one hand of a dog's heart, which, while having 

 ceased beating but not yet become rigid, has been filled with blood 

 at a moderate pressure, and on the other hand of a heart of the 

 same size in which a condition simulating systolic contraction has 

 been brought about by immersing the empty heart in a saturated 

 solution of potassium bichromate at 50 C. The former is taken to 

 represent the diastolic and the latter the systolic form of the heart; 

 and the results are checked by measurements taken between marks 

 placed on various points of the surface of the heart as well as by 

 sections of a heart filled with blood and hardened in a cold solu- 

 tion of potassium bichromate and of one emptied and hardened in 

 the same solution warmed to 50. A comparison of the two hearts 

 in these different conditions tends to shew that while both the 

 right-to-left and antero-posterior diameters are diminished during 

 systole, especially in the plane of the ostia venosa (whereby the 

 auriculo-ventricular orifices become narrowed) the longitudinal 

 diameter, at all events of the left ventricle, is not lessened, the 

 distance between the apex and the auriculo-ventricular groove 

 remaining unchanged. The right ventricle, the change of form of 

 which is complicated, does shorten to a certain extent, and there 

 is during systole a downward movement of the con us arteriosus 

 upon the plane of the ventricular base (which possibly may explain 

 the movement of the needle B in the above mentioned experiment) 

 so that the distance between the apex and the upper border of the 

 conus is less during systole than during diastole. This method 

 also confirms the view that the left ventricle in systole turns on 

 its long axis, towards the right, the movement increasing from the 

 base downwards so that the groove between the two ventricles 

 forms a closer spiral than during diastole. 



Objections may be brought against this method also, and it 

 seems impossible to explain the movements of a lever placed upon 

 the heart unless we admit that during systole, the antero-posterior 

 diameter, of the middle portion of the ventricle at least is increased 

 instead of lessened. We may however probably go so far as to 

 conclude that as far as the ventricles are concerned the chief change 

 during systole is one from a roughly hemispherical to a more 

 conical form, effected without any marked diminution of the 

 distance between the apex and the ventricular base. 



Cardiac Impulse. If the hand be placed on the chest, a 

 shock or impulse will be felt at each beat, and on examination 



