234 THE CARDIAC IMPULSE [Boom. 



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 by the shortening of the side-to-side diameter and the increase 

 of the front-to-back diameter 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. If any shortening 

 does take place it must be largely compensated by the elon- 

 gation 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 during the systole 

 somewhat twisted round, 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. 



Broadly speaking then during systole 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 circular 

 base. 



129. 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. 

 by making an incision through the diaphragm from the abdo- 

 men, and placing the finger between the chest-wall and the 

 apex. It then can be distinctly recognized as the result of the 

 hardening of the ventricle during the systole. And the impulse 

 which is felt on the outside of the chest is chiefly the effect of 

 the same hardening of the stationary portion of the ventricle 

 in contact with the chest-wall, transmitted through the chest- 

 wall to the finger. In its flaccid state, during diastole, the 

 apex is (in a standing position at least) at this point in contact 



