262 CIRCULATION OF THE BLOOD. 



The precise time between the successive acts above described 

 may be measured by arranging a lever of the second order in 

 such a way that, while it rests near its bearings on the con- 

 tracting heart, and follows its movements, its distal end in- 

 scribes those movements on the cylinder of the recording ap- 

 paratus. In this way a tracing is obtained (Fig. 229), in 

 which the relaxation of the heart is marked by a rapid descent 

 of the lever, the auricular contraction l>y a first ascent, the 

 commencement of that of the ventricle by a second, and its 

 continuance by a slow subsidence, suddenly ending in the 

 rapid diastolic descent already mentioned. Thus, in the ex- 

 ample given, the interval between the vertical lines a and b cor- 

 responds to the auricular systole ; that between b and c to the 

 contraction of the ventricle so that the auricles are in dia- 

 stole from 6 to a, the ventricles from c to b. 



58. Study of the Movements of the Heart in Mam- 

 malia. For this purpose a rabbit must be completely clilo- 

 ralized. The trachea having been connected with the appa- 

 ratus for artificial respiration, and the frequency and quantity 

 of the inflations carefully regulated, the chest is opened in the 

 manner already indicated in 49. The facts to be studied are 

 the following : a. At the beginning of the period of relaxation, 

 the heart is so flaccid that it obeys the law of gravitation, and 

 is consequently flattened from side to side, just as we usually 

 see it in the dead bod}'. It does not follow, from this observa- 

 tion, that the relaxed heart has the same form when inclosed 

 in the tligrax. but on other grounds it probably is so, for its 

 form within the chest when in the flaccid condition is mani- 

 festly determined partly by gravity, partly by the shape of the 

 space in which it is contained; and inasmuch as the space is 

 ' a wedge-shaped one, bounded anteriorly by the sternum and 

 ribs, posteriorly by the diaphragm, but virtually unlimited 

 towards either side, we may be quite sure that the organ is at 

 least as much flattened antero-posteriorly in the natural state, 

 as it is seen to be when the chest is open. b. During the re- 

 mainder of the diastole the ventricles are still flaccid and per- 

 fectly passive, but the conditions are changed. While gradu- 

 ally filling with blood, they go through those changes of form 

 which are exhibited by a bladder contained in a basin when it 

 is gradually filled with water, c. At the end of diastole fol- 

 lows a very short period, during which, although the ventricles 

 are still soft, active muscular movements can be observed. 

 This is known as the pra3-systolic period. Systole has in. re- 

 ality begun ; but the auriculo-ventricular valves not having 

 yet had time to close, the ventricular contraction is unresisted. 

 The heart, like any other muscle, so long as it contracts with- 

 out opposition, is soft. d. The moment that the valves close, 

 the heart hardens and becomes globular, slightly twisting 



