CIRCULATION. 473 



minal, their experimental closure enables us to study the effects of the sudden 

 stopping of the blood-supply (ischaemia) of the heart muscle upon the action 

 of the heart. 



Results of Closure of the Coronary Arteries. The sudden closure of one 

 of the large coronary branches in the dog has as a rule either no effect upon 

 the action of the heart beyond occasional and transient irregularity, 1 or is fol- 

 lowed after the lapse of seconds, or of minutes, by the arrest of the ventricu- 

 lar stroke, the ventricle falling a moment later into the rapid, fluttering, 



FIG. 123. A, curve of intra-ventricular pressure, written by a manometer connected with the interior 

 of the left ventricle; B, atmospheric pressure; C, time in two-second intervals. At the first arrow the 

 ramus circumflexus of the left coronary artery was ligated; at the second arrow the heart fell into fibril- 

 lary contractions. The lessening height of the curve shows the gradual diminution of the force of con- 

 traction after ligation. The rise of the lower line of the curve above the atmospheric pressure indicates 

 a rise of intra-ventricular pressure during diastole. The small efevations in the pressure-curve after the 

 second arrow are caused by the left auricle., which continued to beat after the arrest of the ventricle 

 (Porter, 1893). 



undulatory movements known as fibrillary contractions and produced by the 

 inco-ordinated, confused shortenings of individual muscle-cells, or groups of 

 cells. The auricles continue to beat for a time, but the power of the ventricles 

 to execute co-ordinated contractions is lost. 



The Frequency of Arrest. The frequency with which closure is fol- 

 lowed by ventricular arrest depends on at least two factors namely, the size 

 of the artery ligated and the irritability of the heart. That the size of the 

 artery is of influence appears from a series of ligations performed on dogs, 

 arrest being never observed after ligation of the arteria septi alone, rarely 

 observed (14 per cent.) with the right coronary artery, more frequently (28 

 per cent.) with the descendens, and still more frequently (64 per cent.) with the 

 arteria circumflexa. 2 The irritability of the heart is an important factor. In 

 animals cooled by long artificial respiration, or by section of the spinal cord at 

 its junction with the bulb, the ligation of the descendens arrests the heart less 

 frequently than in vigorous animals which have been operated upon quickly. 

 The frequency of arrest is increased by the use of morphia and curare. 3 



Changes in the Heart-beat. Ligation destined to arrest the heart is fol- 

 lowed almost immediately by a continuous fall in the intra-ventricular pressure 

 during systole and a gradual rise in the pressure during diastole (see Fig. 123). 

 The contraction and relaxation of the ventricle are, often slowed. The force 

 of the ventricular stroke is diminished. As arrest draws near, irregularities in 

 the force of the ventricular beat are seldom absent. 4 The frequency of beat is 

 sometimes unchanged throughout, but is usually diminished toward the end ; 



1 The changes produced by subsequent degeneration are not considered here. 



2 Porter, 1893, p. 131. 3 Ibid., 1896, p. 49. * Ibid., 1893, p. 133. 



