CIRCULATION OF THE BLOOD. 



upon the heart-beat ? Compare this with the strong vagus stim- 

 ulation. 



Stimulate the ventricle directly with a tetanizing current and 

 note the change in the character of the contractions. Compare the 

 feel of the heart in this condition with that of the normally beating 

 heart. After the cessation of the exciting cause of the fibrillary 

 contractions, the rabbit's heart will again pulsate normally. The 

 heart of the dog will continue to fibrillate until death, unless it is 

 removed and perfused with defibrinated blood or saline solution. 



Excise the heart. Does it still continue to beat ? Sever the ven- 

 tricles from the auricles by a cut below the auriculo-ventricular 

 groove. Do the severed parts continue to pulsate ? If the ventri- 

 cles have stopped, are they still irritable to mechanical stimuli ? 

 Sever the auricles from each other. Do they still continue to beat ? 



XVIII. ACTION OF THE HEART VALVES. 



The following simple scheme may be used to demonstrate the 

 action of the semilunar valves (see Fig. 32). A dog's heart or a 



J-A 



FIG. 32. Apparatus to Show Action of Semilunar Valves. (Description in text.) 



fresh pig's heart nlay be used. A tube, T, with a smoothly cut end 

 over which is cemented a flat piece of glass, is connected through 



[I0t] 



