ISO PHYSIOLOGY CHAP. 



checked by the contraction of the external and internal coats, 

 which compress the middle coat from above downwards. The 

 longitudinal diameter of the left ventricle thus remains almost 

 unaltered (Krehl). 



The cavity of the right ventricle is reduced in the maximal 

 systole to a narrow space, which is curved towards the left ventricle 

 on account of the convexity of the septum (Fig. 55). Owing to 

 the absence of a middle layer, the longitudinal diameter of the 

 right ventricle is bound to shorten, and contributes to the conical 

 shape assumed by the heart, the apex becoming almost ventrical 

 to the centre of the base. The numerous trabeculae with which 

 the inner layer of the right ventricle is provided, and which 

 connect its walls with the septum, must help to bring ventricle 

 and septum together, and produce an almost complete occlusion of 

 the cavity. 



Besides changes of form we have to consider those of position 

 and volume, which are brought about in systole. 



It is easy to see by direct observation of the exposed living 

 heart that the systolic shortening of its longitudinal diameter 

 occurs not by lifting the apex, but by dropping the base. Haycraft 

 (1891) demonstrated this on the closed thorax of cat and rabbit by 

 pushing needles into the heart, which acted as levers, their fulcrum 

 being in the wall of the thorax. The end of the needle fixed in 

 the base of the heart oscillated upwards at each systole, showing 

 that the base in which it was plunged had sunk. The needles 

 fixed lower down oscillated in a less degree. Lastly, if pushed 

 into the apex, the needle trembled but slightly, showing this to be 

 the point that undergoes least shifting in systole from above down- 

 wards, so that the shortening of the long diameter (which, as we 

 have shown, is due to the longitudinal fibres of the right ventricle) 

 is practically compensated by the downward movement of the 

 base. 



The apex, however, presses a little on the thoracic wall, either 

 because the heart assumes a conical shape, or because the base 

 not only sinks during the emptying of the ventricle but is also 

 tilted a little more obliquely from the back forwards (Carlile and 

 Ludwig). 



Along with the mechanical effects of the cardiac cycle it is 

 necessary, lastly, to consider the changes of volume produced in 

 the heart during this revolution. From what has already been 

 said it is evident that the volume of the heart diminishes during 

 systolic evacuation, and increases during perisystolic filling. It 

 is further apparent that the state of maximum evacuation and 

 minimum volume (which Ceradini proposes to call meiocardia) 

 coincides with the termination of systole, and that the moment of 

 maximal filling and maximal volume (auxocardia) coincides with 

 the close of perisystole. 



