CIRCULATION OF THE BLOOD. 503 



To expose the heart of a chloralized frog, the integument is divided over the 

 sternum and the anterior wall of the upper part of the visceral cavity, which is 

 then to be carefully opened so as to expose the pericardium, taking care not to 

 wound the large abdominal vein. The pericardium is then carefully slit up and 

 the pulsating heart exposed. A large glass rod is now thrust down the oesophagus 

 of the animal so as thoroughly to distend the parts and bring the heart more 

 prominently forward. 



On directing attention to the series of movements which constitute a 

 cardiac revolution, taking first the anterior view alone, the wave of con- 

 traction is distinctly seen to commence in the auricles. The auricles be- 

 come distended and full of blood ; they suddenly contract and transfer 

 their contents into the empty and relaxed ventricle, which is now gorged 

 with blood, but it is not until the auricular contraction is complete that 

 the ventricle, in its turn, contracts and discharges the blood into the 

 relaxed bulbus arteriosus, which becomes distended and finally forwards, 

 in its turn, its contents to the arterial sj'stem. From the fact that while 

 the ventricle contracts the auricles are already filling, and that the ven- 

 tricle does not contract until the auricular contracting is complete, it 

 would appear, on superficial examination, as if there were a constant inter- 

 change of contents between the ventricle and auricles. But on lifting up 

 the ventricle and dividing the little band which connects its posterior sur- 

 face with the pericardium, so as to expose the posterior portions of the 

 heart, the true sequence of contraction is immediately evident. The 

 wave of muscular contraction distinctly originates in the sinus venosus, 

 extends to the auricles, and it is not until their contraction is complete 

 that the ventricle contracts ; but by this time the sinus is again already 

 full and the auricles filling. Therefore, while the ventricular contraction 

 is determined by that of the auricles, and the auricular by that of the 

 sinus, the contraction of the latter originates independently of an y previ- 

 ous movement. 



This sequence of contraction may be graphically represented by resting a 

 lever of the second class on the contracting heart in such a manner that its point 

 will describe its movements on the 

 smoked surface of a revolving drum, 

 as show/i in the accompanying tracing 

 (Fig. 197) : 



In this curve the first slight ascent 

 represents the contraction of the 

 auricles, and its subsidence represents 

 the interval between its contraction 

 and that of the ventricle, thus showing 

 that the one is complete before the 

 other commences. The next more de- 

 cided ascent is due to the vigorous con- FlG \ I 9 J'T TRA ^ TNG DRAWN BY A LEVER 



. * / ., i * t T *i i -/Vrx*lMEL) UIRKCTIjY TO THE APEX. OF 



traction of the ventricle and the descent THE FROG'S HEART. (Sanderson. ) 



tO its relaxation We learn from this x contraction of auricles; B, contraction of ventricle; 



that the auricles have completed their c, relaxation of ventricle, 

 systole before the ventricle contracts. 



Passing now to the movements of the heart as seen in mammals the thorax is 

 to be opened in a rabbit, rendered insensible by chloral, by dividing the integu- 

 ment from the larynx to the end of the xiphoid cartilage and then removing the 



