CHAP, iv.] THE VASCULAR MECHANISM. 187 



valid. And, indeed, it is maintained by some that the front-to- 

 back diameter does actually diminish during systole. 



But it is at least clear that the front-to-back diameter, even if 

 it does not increase, diminishes far less than does the side-to-side 

 diameter ; and hence during the systole there is a change in the 

 form of the section of the base of the ventricles. During the 

 diastole this has somewhat the form of an ellipse with the long 

 axis from side to side, but with the front part of the ellipse much 

 more convex than the back, since the back surface of the ventricles 

 is somewhat flattened. During the systole this ellipse is converted 

 into a figure much more nearly resembling a circle. It is urged, 

 moreover, that the whole of the base is constricted, and that the 

 greater efficiency of the auriculo-ventricular valves is thereby 

 secured. 



As to the behaviour of the long diameter from base to apex, 

 observers are not agreed ; some maintain that it is shortened, and 

 others that it is practically unchanged. And, in any case, a change 

 in this diameter plays little or no part in the expulsion of the 

 contents of the ventricle ; this expulsion is effected by the contrac- 

 tion of the more transversely disposed fibres, whereby the cavity is 

 reduced to an elongated slit. Moreover, if any shortening does take 

 place it must be compensated by the elongation of the great vessels, 

 which, as stated above, may be seen in an inspection of the beating 

 heart. For there is evidence that the apex, though, as we have 

 seen, it is somewhat twisted round during the systole, and at the 

 same time brought closer to the chest-wall, does not change its 

 position up or down, i.e. in the long axis of the body. If in a 

 rabbit or dog a needle be thrust through the chest-wall so that its 

 point plunges into the apex of the heart, though the needle 

 quivers, its head moves neither up nor down, as it would do if its 

 point in the apex moved down or up. 



During systole, broadly speaking, the ventricles undergo a 

 diminution of total volume, equal to the volume of contents 

 discharged into the great vessels (for the walls themselves like all 

 muscular structures retain their volume during contraction save 

 for changes which may take place in the quantity of blood 

 contained in their blood vessels, or of lymph in the intermuscular 

 spaces), while they undergo a change of form which may be 

 described as that from a roughly hemispherical figure with an 

 irregularly elliptical section to a more regular cone with a more 

 nearly circular base. 



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

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

 this impulse, ' cardiac impulse,' will be found to be synchronous 

 with the systole of the ventricle. In man, the cardiac impulse may 

 be most distinctly felt in the fifth costal interspace, about an inch 

 below and a little to the median side of the left nipple. In an 

 animal the same impulse may also be felt in another way, viz. 



