2IO ESSENTIALS OF PHYSIOLOGY. 



consists of small spindle-shaped fibres showing cross striation, which is 

 very indistinct, particularly in the sinus venosus. 



Two nerves, the right and left vago-sympathetic nerve trunks, enter 

 the heart at the sinus, and become connected with a small mass of 

 nerve cells which lies close to the sino-auricular junction and is known 

 as Remak's ganglion. Nerve fibres from this ganglion pass along the 

 septum between the auricles to enter two similar ganglia (Bidder's 

 ganglia), lying close to the auriculo- ventricular junction. Scattered 

 nerve cells are also found in the interauricular septum and in the upper 

 part of the ventricle, but are absent from its apical half. The fibres 

 issuing from all these groups of nerve cells terminate in relation 

 with the muscular fibres of the heart. 



THE BEAT OF THE FROG'S HEART. 



If the brain of a frog is destroyed and the heart exposed, it can be 

 seen that each beat consists of a regular sequence of events, namely, (1) 

 contraction of the sinus, followed by that of (2) the auricles, (3) the 

 ventricle, and finally (4) the bulbus arteriosus. When the whole heart 

 is carefully excised from the body and placed in a watch glass containing 

 salt solution (0*65 per cent. NaCl), it continues to beat in a normal 

 fashion for some time. On separating the sinus from the rest of the 

 heart, by cutting through the sino-auricular junction, the sinus 

 continues to beat as vigorously and at the same rate as before, whereas 

 the auricles and ventricle cease to beat. 



After a short time the auricles and ventricle again begin to beat, 

 but at a slower rate than the sinus. If the ventricle is cut away from 

 the auricles, the latter continue to beat, while the ventricle after one 

 or two beats usually comes to a standstill. After an interval of half 

 an hour or more the ventricle may again begin to beat, and it will do 

 so more readily if it is stimulated by an occasional pin-prick. The rate 

 of the ventricular beat is slower than that of the auricles. The apical 

 half of the ventricle, if isolated, will never again start to beat of its own 

 accord. This experiment makes it clear, first, that the rhythmic beat 

 of the heart can be carried on quite independently of the central nervous 

 system, and secondly, that this power of rhythmic contraction is most 

 fully developed in the sinus. 



It was formerly supposed that the beat originated in the nerve cells 

 of the heart, from which a constant stimulus was sent out to the heart 

 muscle, and that the muscular fibres responded to this stimulus by a 

 series of rhythmic contractions. This is the neurogenic theory of the 

 cause of the heart beat. The view now generally held, however, is that 



