544 CIRCULATION OF BLOOD AND LYMPH. 



consists of a mass of nodal tissue lying in the wall of the right 

 auricle and possessing connections with the musculature of the 

 right auricle and the right ventricle. When all parts of the auriculo- 

 ventricular groove (including the A-V bundle) are severed, except 

 the right lateral wall at the location of this bundle of Kent,* beats 

 originating in the auricle are transmitted to the ventricle in due 

 sequence. It seems probable that this additional connection 

 between auricle and ventricle, designated as the right lateral 

 connection, may play some role, as yet undetermined, in the 

 maintenance of the sequence of the heart beat. 



The Sequence of the Heart Beat. In the rhythmic heart beat 

 it is believed that an excitation or impulse arises first in the sino- 

 auricular node and thence spreads over the heart in the form of a 

 wave or pulse. The wave of excitation is followed by a wave of 

 contraction. That the excitation starts in the sino-auricular node 

 is indicated by the fact that the first appearance of negativity, 

 as detected by the galvanometer (see following paragraph), occurs 

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

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

 ture elsewhere are without effect. Direct efforts to demonstrate 

 this function of the S-A node by the method of extirpation have 

 given conflicting results. The course of the wave of excitation 

 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 expression, as its 

 pace-maker. From the S-A node the excitation spreads in a uni- 

 form 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 auriculo-ventricular node through the auric- 

 ular musculature, and in the node suffers some delay, owing prob- 

 ably 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 



* Kent, "Proceedings of the Royal Society," B. 87, 198, 1914, and" Journal 

 of Physiology," 48, 1914, "Proc. Physiol. Soc." 



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

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



