526 CIRCULATION OF BLOOD AND LYMPH. 



end is beautifully shown in the frog's heart, in which the contraction 

 begins in the sinus venosus, spreads to the auricles, thence to the 

 ventricle, and finally to the bulbus arteriosus. Under normal 

 conditions this sequence is never reversed, and an explanation of 

 the natural order forms obviously an important part of any complete 

 theory of the heart beat. Those who hold to the neurogenic theory 

 naturally explain the sequence of the beat by reference to the 

 intrinsic nervous apparatus. If the motor ganglia lie toward the 

 venous end of the heart one can imagine that their discharges may 

 affect the different chambers in sequence, the pause between 

 auricular and ventricular contraction being due, let us say, to the 

 fact that the motor impulses to the ventricle have to act through 

 subordinate nerve cells in the auriculo-ventricular region, and the 

 time necessary for this action brings the ventricular contraction 

 a certain interval later than that of the auricle. There is no 

 immediate proof or disproof of such a view. The numerous exper- 

 iments made upon the rapidity of conduction of the wave of 

 contraction over the heart are not conclusive either for or against 

 the view. The fact, however, that in the quiescent but still irritable 

 heart the rhythm may be reversed by artificially stimulating the 

 ventricle first seems to the author to speak strongly against the 

 dependence of the sequence upon any definite arrangement of 

 neuron complexes. On the myogenic theory the sequence of the 

 heart beat is accounted for readily by relatively simple assumptions. 

 Gaskell and Engelmann have each laid emphasis upon the facts in 

 this connection, and the application of the myogenic theory to the 

 explanation of the normal sequence of contractions forms one of its 

 most attractive features. Gaskell assumes * that the rhythmical 

 power of the muscle at the venous end is greater than that at the 

 ventricular end, that is, if pieces from the two ends are examined 

 separately it will be found that the spontaneous rhythm of the 

 tissue from the venous end is more rapid. This portion of the 

 heart, therefore, beating more rapidly, sets the rhythm for the 

 whole organ, since a contraction started at the venous end will 

 propagate itself from chamber to chamber. The pause between the 

 contractions of the successive chambers between auricle and 

 ventricle, for instance is due, in the heart of the tortoise, to the 

 fact that the muscular tissue at the junction of auricle and ventricle 

 has a relatively low rate of conduction. At this point, indeed, 

 the muscular fibers form a ring around the orifice, preserving, 

 therefore, the arrangement found in the embryo at the time that 

 the heart has the form of a tube. Gaskell has given reasons for 

 believing that the conduction of the wave of contraction is slower 



* Gaskell, "Journal of Physiology," 4, 61, 1883; also vol. ii, p. 180, of 

 Schafer's " Text-book of Physiology," 1900. 



