148 THE CIRCULATION OF THE BLOOD AND LYMPH 



differentiated into fibrillae. They seem to represent the remains of 

 the primitive cardiac tube, which by the development of certain 

 pouches and twists becomes transformed into a multi-chambered 

 heart. Their resemblance to embryonic fibres suggests that they 

 may have retained the primitive capacity of the mesodermic tissue 

 of the embryonic heart to conduct, and even to originate, the 

 rhythmical contraction. But while there is no decisive evidence 

 that they constitute an automatic cardio-motor centre, as some 

 authors have supposed, they, or at least the narrow bridge of tissue 

 in which they lie, do play an important part in the conduction of 

 the contraction from the auricles to the ventricles. For compres- 



Fig. 64. Right Auricle and Ventricle of Calf, to show Auriculo-Ventricular Band 

 (Keith), i, central cartilage; 2, main auriculo-ventricular bundle; 3, auriculo- 

 ventricular (A-V) node; 4, right septal division of the bundle ; 5, moderator band; 

 6. medial or septal cusp of tricuspid valve ; 8, coronary sinus. 



sion of the band produces a block, just as the pressure of a clamp 

 in the auriculo-ventricular groove does in the frog's heart (Kent). 

 With a certain degree of pressure the ventricle beats only once for 

 two beats of the auricle, with greater pressure only once for three 

 or more auricular beats. With a still greater pressure or after 

 crushing or section of the bundle conduction is abolished, and the 

 ventricle either remains at rest for a time, as in the frog's heart, or, 

 what is much more common, immediately starts beating with an 

 independent rhythm, which is slower than that of the auricles 

 (Erlanger). It can be considered certain that in these observations 

 nerves may have been involved in the block as well as the muscle of 



