THE HEART BEAT. 537 



at this point, and by the further fact that coohng in this region 

 slows the rhythm of the heart beat, while variations in tempera- 

 ture elsewhere are without effect. When the sino-auricular node 

 is removed by surgical operation the initiation of the heart beat 

 is transferred to the auriculo ventricular node. An auriculo- 

 vcntricular rhythm is established in which the two chambers beat 

 simultaneously or nearly so.* The course of the waVe of excita- 

 tion from the S-A node over the heart has been studied in various 

 ways, but with not wholly concordant results. According to a 

 recent summary given by Lewis, f and based largely on his own 

 work, the sequence of events is as follows: The excitation starts 

 in the S-A node, which possesses the highest rhythmicity of any 

 part of the heart, and serves, therefore, to use a figurative expres- 

 sion, as its pace-maker. From the S-A node the excitation spreads 

 in a uniform manner over the auricles, traveling with a velocity 

 of about 1000 mms. per second, and the following contraction 

 takes the form of a rapidly moving peristaltic wave which forces 

 blood toward the ventricle. The contraction spreads backward 

 also for some distance along the cavse and the pulmonary veins. 

 The excitation or impulse reaches the auriculoventricular node 

 through the auricular musculature, and in the node suffers some 

 delay, owing probably to the slower rate of conduction exhibited 

 by this tissue. It is this delay at this point which causes mainly 

 the perceptible interval between the auricular and the ventricular 

 contraction. From the A-V node the excitation is conveyed to 

 the ventricle through the conducting system, that is, the auriculo- 

 ventricular bundle and its ramifications over the internal surface 

 of the two ventricles. In this tissue the conduction is very rapid, 

 about 3000 to 5000 mms. per second. By means of this system of 

 so-called Purkinje fibers the excitation is distributed nearly uni- 

 formly to the musculature of the two ventricles. It was the older 

 view that the excitation and contraction reached first some defi- 

 nite region and thence followed the anatomical course of the fibers, 

 but according to Lewis the excitation is conveyed by the conduct- 

 ing system almost simultaneously to the whole of the interior 

 surfaces of the ventricles, and at each point radiates out through 

 the muscular mass toward the external surface. The result is that 

 the whole mass of the musculature contracts practically at once, 

 although careful galvanometric examination of the external sur- 

 face by the method referred to in the next paragraph reveals that 

 the excitation reaches the surface first where the musculature is 

 thinnest or is connected most directly with the A-V bundle. 



*Eysterand Meek, "Physiological Reviews," 1, 1, 1921. This paper 

 contains a complete review to date, with references to literature. 



t Lewis, Croonian Lecture, "Proceedings of the Royal Society," London, 

 B. 89, 560, 1917; also "Harvey Lectures," 1914-15. 



