CIRCULATION OF THE BLOOD. 



Compare this with the response of skeletal muscle to stimuli of 

 various strengths. 



XIII. REFRACTORY PERIOD. 



Connect a frog's heart with the cardiograph lever. Let this re- 

 cord upon a medium rapid drum. Arrange a signal magnet in the 

 primary circuit, with the point of its writing lever placed directly 

 under the point of the cardiograph lever. 



Stimulate the ventricle at intervals of ten or fifteen seconds, 

 with maximal make or break shocks from the inductorium. At- 

 tempt to stimulate each time at a different part of the heart's cycle. 



Some of the stimuli will be accompanied by an extra-contrac- 

 tion ; others will have no effect so far as calling forth an extra-ven- 

 tricular contraction is concerned. The first stimuli were applied 

 during the irritable stage of the heart muscle. The second were 

 applied during its refractory period. Where an extra-contraction 

 occurs, it is followed by a pause, known as the compensatory paust, 

 since it generally restores the rhythm which prevailed before the 

 extra-contraction occurred. 



Systematically stimulate the ventricle at different periods of the 

 cycle and note your results on the drum, thus mapping out the 

 limits of the refractor}- period. 



XIV. DISSECTION OF MAMMALIAN HEART. 



Open the thorax of a dead rabbit. Note the position of the heart 

 in the pericardium and its relation to the surrounding viscera. 

 Note the reflections of the pericardium over the great vessels which 

 enter and leave the heart. Open the pericardium and note the po- 

 sition of the heart in the thorax. What part of the heart is in clos- 

 est relation to the chest wall ? What is the position of the right and 

 left ventricles in relation to the anterior thoracic wall ? In relation 

 to the diaphragm ? Note the number of auricles and ventricles in 

 comparison with the frog's heart. Also note the more distinct dif- 

 ferentiation of the pulmonary system in the mammal as compared 

 with the frog. 



7 [97] 



