ELEMENTARY EXPERIMENTAL PHYSIOLOGY 95 



a short period of inhibition both parts begin to beat, but with a 

 different rhythm. The sinus is the more injured, and beats at a slower 

 rate. If the cut be made through the auricles, the sinus beat continues 

 and is unaffected by the injury. Cut off the ventricle just above the 

 auriculo-ventricular groove. After a period of inhibition both auricles 

 and ventricle beat. The auricles recover first. Cut through the ventricle 

 below the auriculo-ventricular groove. The apex preparation does not 

 beat spontaneously. It responds to a prick by a beat, and may in some 

 cases be taught to beat rhythmically by rhythmic stimulation. A 

 crystal of common salt placed on the apex or the passage of the 

 galvanic current through the apex preparation provokes its rhythmic 

 contraction. 



Cut out small pieces of the bulbus arteriosus, and place them under 

 the microscope in a watch glass containing Ringer's fluid. The pieces 

 will beat rhythmically. There are few if any nerve cells in the bulbus, 

 and there are certainly none in some of these pieces, so the rhythm is 

 probably the function of the heart muscle. In support of this are the 

 following facts : 



A frog's heart painted with nicotine (1 per cent, solution) continues 

 to beat. Nicotine paralyses nerve cells. 



Isolated portions of the mammalian heart will beat rhythmically 

 for hours if fed through their nutrient arteries with oxygenated blood. 



The structural elements of the heart are nucleated, branched, and 

 cross-striated cells. The muscle-cells are joined together into net- 

 works and bands, so as to form one functional whole, and hence 

 excitation of any one part leads to the contraction of the whole. The 

 first part to begin to functionate in the embryo is the venous end. In 

 the mammalian heart it has been shown that muscle fibres of a 

 peculiar type connect the auricles with the ventricles ; they form the 

 auriculo ventricular bundle. 



The above experiments suggest that rhythmic contractility is the 

 inherent function of the cardiac muscle. The muscle of the sinus and 

 auriculo-ventricular junction is more embryonic in structure and possesses 

 greater power of initiating rhythm. It is less excitable, and conducts 

 a stimulus less rapidly than the muscle of the auricles and ventricle. 

 The auricular and ventricular muscle is more differentiated in structure. 

 The cross striae are more marked. It does not so easily initiate 

 rhythm. Owing to its greater excitability and conductivity it follows 

 the lead of the sinus. 



During the period of systole the heart is refractory to artificial 

 excitation. The excitability returns with diastole, increasing as 



