278 THE MECHANICS OF THE HEART 



auricles contract together, accurate measurements have shown that 

 the left one lags somewhat behind the right. The interval, of course, 

 is extremely brief; it amounts to only 0.01 to 0.03 sec. The excitation 

 wave finally reaches the papillary bases of the ventricles by way of the 

 bundle of His and its distal ramifications. The wave itself is accur- 

 ately timed so that a perfect coordination between the different seg- 

 ments of the heart is assured. We have previously noted that a period 

 of almost 0.2 sec. elapses before the wave arrives in the distalmost 

 muscle strands of the ventricle, but naturally, the conduction is not 

 equally rapid in all parts of the heart. Thus, it has been found that 

 the bundle of His conducts rather slowly, because the wave attains 

 here a velocity of only 10 to 15 cm. in a second. This fact is of interest, 

 because, as has previously been shown, the ventricle contracts after 

 the auricle, the interval between their systoles amounting to 0.12-0.2 

 sec. Hence, the resistance in this bridge of tissue has been adjusted 

 in such a way that a perfect sequence of contraction is obtained. 



Two views are held regarding the manner in which the ventricular 

 musculature is activated. It was formerly believed that the segments 

 situated nearest the auriculoventricular groove, contract first, while 

 those closest to the apex are involved last. The results of electrical 

 measurements and of cinematographic records of the beating heart, 

 taken by Nicolai and others, 1 however, have shown that the excitation 

 wave is conducted directly to the papillary projections, and hence, it 

 must be concluded that this particular system of the ventricles is 

 activated first. The contraction wave spreads from here to the oblique 

 and circular muscle fibers. Clearly, this view is entirely in accord with 

 the anatomical arrangement of the conducting path, because, as has 

 been stated above, the main bundle of His is enveloped in a sheath of 

 fibrous tissue, while its terminals, the fibers of Purkinje, are directly 

 traceable to the papillary muscles. 



Heart-block. The preceding statements find amplification in the 

 observations of Gaskell, 2 showing that the passage of the wave of excita- 

 tion through the hearts of frogs and turtles may be greatly retarded by 

 compressing the tissue forming the auriculoventricular groove. While 

 this end may be attained with the help of a pair of forceps, a better way 

 is to adjust a screw-clamp to this region, which enables the experimenter 

 to grade the pressure more accurately and to obtain different degrees 

 of blocking. Under ordinary conditions every contraction of the auri- 

 cles is followed by a contraction of the ventricles, because the wave 

 of excitation meets with no obstacle in its passage through the bundle. 

 If the latter is now thoroughly compressed by the closure of the clamp, 

 the impulse is blocked at the seat of the injury and cannot reach the 

 ventricles. This particular segment of the heart now ceases to beat, 

 while the auricular portion continues its activity as previously estab- 



1 Braun (Herzbewegung und Herzstoss, Fisher, Jena, 1898), and Rehfish 

 (Berliner klin. Wochenschr., 1908, Nr. 26). 



2 Jour, of Physiol., iv, 1883, 66. 



