THE HEAKT. 279 



very perceptible to the eye at each pulsation of the heart, when 

 exposed in the living animal. 



4. The protrusion of the point of the heart at the time of con- 

 traction, together with its rotation upon its axis from left to right, 

 brings the apex of the organ in contact with the parietes of the 

 chest, and produces the shock or impulse of the heart, which is 

 readily perceptible externally, both to the eye and to the touch. 

 In the human subject, when in an erect position, the heart strikes 

 the chest in the fifth intercostal space, midway between the edge 

 of the sternum and a line drawn perpendicularly through the left 

 nipple. In a supine position of the body, the heart falls away from 

 the anterior parietes of the chest so much that the impulse may 

 disappear for the time altogether. This alternate recession and 

 advance of the point of the heart, in relaxation and contraction, 

 is provided for by the anatomical arrangement of the pericardium, 

 and the existence of the pericardial fluid. As the heart plays back- 

 ward and forward, the pericardial fluid constantly follows its 

 movements, receding as the heart advances, and advancing as the 

 heart recedes. It fulfils, in this respect, the same purpose as the 

 sy no vial fluid, and the folds of adipose tissue in the cavity of the 

 large articulations ; and allows the cardiac movements to take place 

 to their full extent without disturbing or injuring in any way the 

 adjacent organs. 



5. The rhythm of the heart's pulsations is peculiar and somewhat 

 complicated. Each pulsation is made up of a double series of con- 

 tractions and relaxations. The two auricles contract together, and 

 afterward the two ventricles ; and in each case the contraction is 

 immediately followed by a relaxation. The auricular contraction 

 is short and feeble, and occupies the first part of the time of a 

 pulsation. The ventricular contraction is longer and more powerful, 

 and occupies the latter part of the same period. Following the 

 ventricular contraction there comes a short interval of repose, after 

 which the auricular contraction agains recurs. The auricular and 

 ventricular contractions, however, do not alternate so distinctly 

 with each other (like the strokes of the two pistons of a fire engine) 

 as we should be led to believe from the accounts which have been 

 given by some observers. On the contrary, they are connected and 

 continuous. The contraction, which commences at the auricle, is 

 immediately propagated to the ventricle, and runs rapidly from the 

 base of the heart to its apex, very much in the manner of a peri- 

 staltic motion, except that it is more sudden and vigorous. 



