136 A MANUAL OF PHYSIOLOGY . 



interventricular septum. The fibres of the bundle are narrower 

 than the other fibres of the auricles, very rich in nuclei, and only 

 slightly 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 sug- 

 gests 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 compression 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 con- 

 duction is abolished, and the ventricle either remains at rest for 

 a time, as in the frog's heart, or, what is much more common, im- 

 mediately 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 the auriculo-ventricular 

 band, since this band is richly provided with nerve-fibres as well 

 as ganglion-cells (Wilson). Yet it is unlikely that all the nerves 

 capable of conducting the impulses to contraction should be 

 gathered into such a narrow compass, and therefore the experi- 

 ment supports the view that the conduction is carried out in the 

 muscular tissue. And if the conduction of the excitation from 

 auricles to ventricles is accomplished by a muscular connection, 

 it is natural to suppose that the co-ordination of symmetrical 

 portions of the heart on either side of the longitudinal axis, the 

 co-ordination in virtue of which the two auricles contract together 

 and the two ventricles together, is also achieved by the passage 

 of impulses through the muscular tissue. In accordance with 

 this, it has been shown that the ventricles in the dog and cat 

 continue to beat in unison, after the attempt has been made to 

 sever any nerves connecting them by extensive zigzag incisions, 

 so long as they are united by a narrow bridge of muscle (Porter) . 



In disease, interference with the conduction of the stimulus from 

 auricles to ventricles along the atrio-ventricular bundle is a not 

 uncommon phenomenon. According to the degree of interference, 

 the ventricular contraction may be simply delayed, or only a certain 

 proportion of the auricular contractions (every second, every third, 

 or every fourth) may be conducted to the ventricle, or, finally, the 

 block may be complete, and the ventricle then contracts quite inde- 



