570 



Mr. T. Lewis. 



the ratio between the rate of conduction through the muscle and through 

 Purkinje tissue. The proportion is approximately as 1 is to 10. 



Again, if the rate of conduction between two points on the surface of the 

 heart is examined and these points lie on the wall of tbe ventricle where this 

 is thickest, the conduction rate is approximately 300-500 mm. per second ; 

 a similar rate of propagation is ascertained during the natural passage of the 

 wave from within outwards through the muscle wall; but if the rate of 

 conduction is tested similarly over thin portions of the ventricular wall, it is 

 found to be as high as 1500-2000 mm. per second. In the last-named 

 circumstances the wave is short-circuited through the Purkinje network for the 

 longest part of its course. After consideration of all the circumstances we have 

 concluded that the rate of conduction through ventricular muscle lies between 

 300-500 mm. per second, while that through straight paths of Purkinje tissue 

 lies between 3000-5000 mm. per second. 



This conclusion clears away the discrepancies in surface readings to which 

 allusion has been made. The surface points which are activated earliest are 

 those points which overlie the thinnest parts of the ventricular walls. There 

 is an unmistakeable relation between any surface reading and the thickness 

 of the muscle at the point tested. 



•The surface readings are controlled by two factors : first by the length of 

 the Purkinje strands and secondly by the thickness of the muscle wall. If a 

 number of surface points are tested, and the heart fixed ; if measurements are 

 made upon the lining and on the basis of the estimated conduction rate over 

 the lining the corresponding times are calculated ; and if measurements are 

 made of the thickness of the ventricular wall and on the basis of the estimated 

 conduction rate in ventricular muscle the corresponding times are calculated 

 and added to those of the first series ; then a complete series of calculated 

 readings is obtained which harmonises with the readings won from the 

 naturally beating heart. 



A General Summary. 



We are now in possession of a very detailed knowledge of the course taken 

 by the contraction wave through the mammalian heart. It behoves us, in so 

 far as we are able, to review the mechanism of the heart beat in the light 

 of this new knowledge. The mammalian heart has onerous duties to 

 perform, but the first plan of its construction is the plan of a simple tube 

 contracting peristal tically from end to end without interruption. During 

 phylogenetic and embryological development the work of the heart increases, 

 the shock of its movements increases. The heart divides in two, an upper 

 and lower chamber, the first frail and thin-walled, the second sturdy and 



