540 CIRCULATION OF BLOOD AND LYMPH. 



stitute, according to Keith and Flack,* a remnant of the original 

 invagination of muscular tissue from the auricular ring (Fig. 224), 

 through which auricle and ventricle are connected in the lower 

 vertebrates. In addition to the auriculoventricular bundle Kent 

 describes two other connections between auricles and ventricles. 

 The more important of these lies on the right side of the heart, and 

 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, f 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 Contraction Wave in the Heart. It seems to be demon- 

 strated that normally the contraction of the heart begins in the 

 sinus tissue of the right auricle the so-called sino-auricular node 

 described in the preceding paragraph, or in the neighboring sinus 

 tissue. The tissue of the sino-auricular node is designated some- 

 times as the pace-maker of the heart, on the assumption that 

 ifcs rhythm sets the pace for the rate of beat of the whole heart. 

 Experiments made to demonstrate this point by excising or de- 

 stroying the portion of the sinus region containing the node have 

 given conflicting results. J Some authors state that the rate of the 

 heart-beat is not affected by such operations, while others claim 

 that the subsequent auriculoventricular beat takes on perma- 

 nently a slower rhythm. 



In the mammalian heart, when exposed to view, it is evident 

 that the auricular systole is not sufficient to empty its cavity, so 

 far at least as the atrium is concerned. The contraction of the 

 auricular appendages is more forcible. The contraction may be 

 regarded as a rapid peristalsis, which sweeps a portion of the blood 

 before it into the ventricle. The force of the contraction has been 

 determined in a number of cases. For the auricle of the dog's 

 heart the pressure caused by the contraction has been estimated 

 to be equal to 9 to 20 mm. Hg. The systole of the ventricle is to 

 the eye a simultaneous contraction of the whole musculature. 

 Various observers, however, have shown that the wave of contrac- 



* Keith and Flack, "Journal of Anat. and Physiology," 41, 172, 1906, and 

 43, p. 1. 



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

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



t See Cohn, Kessel, and Mason, "Heart," 3, 311, 1912, and Moorhouse, 

 "American Journal of Physiology," 30, 358, 1912, and 31, 421, 1913. 



