ii CHANGE OF FORM IN MUSCLE DURING ACTIVITY 163 



i.e. the rapidity with which the excitation is transmitted from 

 section to section. Under normal conditions the velocity is so 

 great in the frog's heart that all points, as in the excitation of 

 striated skeletal muscle, seem to contract simultaneously. This 

 effect may persist in a fresh, vigorous heart, even when the 

 ventricle has been cut up into several pieces. As a rule, how- 

 ever, the undulatory process of the contraction can then at once 

 be recognised. It often seems as though conduction was slower 

 in the bridges than in the larger pieces, for each of the latter 

 seems to contract together, as a whole, while a perceptible time 

 elapses between the contraction of two successive pieces. Engel- 

 mann estimated the average rate of conductivity in strips of 

 muscle 10-15 mm. long (snipped out of the ventricle) at about 

 30 mm. per sec., or more usually 10-20 mm., measured by 

 a metronome, giving beats at ^ sees. Although these values 

 must be considerably below the normal, we may conclude from 

 them that even in the most favourable cases the rate of conduct- 

 ivity must be lower than it is when the excitation is transmitted 

 by nerve. The rate of transmission is diminished to a striking 

 degree by cooling the preparation. Cooling from 17 to 50C., 

 e.g., is sufficient to reduce it from 20 to 8 mm. Under normal 

 conditions the excitation is invariably transmitted from auricle 

 to ventricle. Engelmann (22) has recently ascertained that 

 muscular conduction is in this case the sole factor, by experi- 

 ments on the " suspended " frog's heart, arranged on the same 

 principle as that employed by Helmholtz to measure the rapidity 

 with which the excitation of nerve is transmitted. The auricle to 

 some extent represents the nerve, the ventricle the muscle ; the 

 former was excited at different distances from the ventricle, and 

 the latent period of the ventricular systole in each case was 

 measured. If conduction was effected by nerve-fibres, the short- 

 ness of the strips employed would render any perceptible difference 

 improbable, whereas with cell-conduction in the muscle it was 

 to be expected. As a matter of fact, a very considerable retarda- 

 tion was observed in the commencement of the ventricular 

 systole, when the auricle was excited at a greater distance. In 

 a given case, in which, however, the rapidity was no longer 

 perfectly normal, the delay amounted to 0'09 sec,, corresponding 

 to a rate of conductivity of 90 mm. per sec. But this, as 

 Engelmann pointed out, is a value 300 times lower than the 



