504 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



sternum by cutting through the costal cartilages on each side with a pair of strong 

 scissors. If the operation is performed carefully very little hemorrhage will 

 occur. On dilating the opening in the thorax thus formed, and maintaining the 

 opening by means of hooks fastened to weighted cords, the movements of the 

 heart are readily followed. We have, however, by thus opening the thoracic 

 cavity interfered with respiration, and to permit of a reliable study of the cardiac 

 movements the animal's life must be prolonged by artificial respiration. 



Having now the heart and great vessels well exposed, and the normal con- 

 ditions as regards respiration imitated as nearly as possible, the sequence of events 

 which make up a cardiac revolution may be readily followed. 



Starting in the distended venous trunks at the base of the heart, as 

 seen in the mammal, the wave of contraction extends rapidly to the auri- 

 cles, now filled with blood, which contract with a sudden, sharp systole, 

 discharging their contents into the flaccid ventricles, the auricular append- 

 ages becoming pale and the whole auricle being drawn down toward the 

 auriculo-ventricular ring. During the systole of the auricles, the ventricles 

 become more and more gorged with blood, but when felt are still soft 

 and flaccid. Immediately, however, upon the completion of the auricular 

 contraction, the ventricles harden and become globular, or, in other words, 

 shorten and thicken. Examined more closely, it is seen that during 

 repose the ventricles form an imperfect cone, its base being a transverse 

 ellipse, but during systole they form a more perfect cone, shortened in 

 its long axis, and having a circle for a base, the greatest contraction hav- 

 ing taken place in its longitudinal and transverse axes, while the antero- 

 posterior diameter of the base of the cone has been little altered, although 

 the circumference of the base has been actually increased (Foster). At 

 the moment of ventricular systole the heart rotates on its axis to the 

 right, so as to expose more of the left ventricle, while the apex seems to 

 approach the base. The rotation of the heart is due to the contraction of 

 the muscular fibres which pass from the sternal surface of the auriculo- 

 ventricular rings obliquely downward and to the left ; when they shorten 

 they raise the apex and bring more of the posterior surface of the ven- 

 tricles in relation with the chest-walls. At this moment the aorta and 

 pulmonary artery are seen to expand and lengthen, thus compensating 

 for the shortening in the long axis of the ventricle. Then, as diastole 

 commences, the ventricles flatten and elongate, the great arteries con- 

 tract and shorten, the heart rotates back to the left, and the cardiac revo- 

 lution is completed. 



It has been stated that in the systole the apex seemed to be drawn 

 up toward the base. This, however, is not exactly correct. 



In the conditions in which we have been studying the heart its nor- 

 mal supports have been more or less interfered with, and although we 

 have not thereby lessened the value of the conception obtained of the 

 sequence of contractions, we ma}' perhaps extend our knowledge as to 

 the degree and character of the locomotion performed by different parts 

 of the heart in its contraction. 



