CARDIAC MUSCLE. 439 



All that has been said on the mode of action of battery currents 

 of short closure applies to induction currents. The induction current 

 which is produced in the secondary circuit of a du Bois-Reymond's 

 inductoriurn at closure of the primary circuit, evokes an excitation 

 which has its origin exclusively at the cathode. It has been long known 

 that nerveless muscles react relatively more readily to closure of the 

 voltaic current than to the induction current. This is in accordance 

 with the observation, that in muscles of which the motor nerves have 

 been severed for a sufficient time to produce complete degeneration, 

 even strong induction shocks evoke no contraction, notwithstanding 

 that they respond readily to battery currents of moderate strength. 



In connection with these facts, it may be noted that in unstriped 

 muscles a relation presents itself between duration of closure and 

 response, similar to that in Tick's experiment. The two cases are, 

 however, scarcely comparable. In striped muscle we are concerned 

 with thousandths of a second, in smooth muscle with durations of 

 several seconds ; and that the difference is not one of degree only, but 

 of kind, is probable, for, as we have already seen, there is in striped 

 muscle a second kind of effect introduced when currents of long dura- 

 tion are used. In Engelmann's observations on the ureter of the rabbit, 

 it was found that even with very strong currents a closure of a quarter 

 of a second was required to evoke a response ; and that, if the strength 

 of the current was diminished, it was necessary to close it for a still 

 longer time. Similarly, Biedermann could not obtain a full contraction 

 of the adductor of Anodonta with a shorter closure than one of four 

 seconds. Shorter closures evoked responses, but of much smaller amount. 



CARDIAC MUSCLE. 



In the preceding account of the physiological properties of striped 

 muscular fibre, only occasional reference has been made to that form 

 of it which exists in the heart. 



The physiological properties of cardiac muscular tissue may be dis- 

 cussed in the same way as those of skeletal muscle. It responds 

 similarly to an instantaneous stimulus, and can be excited to contraction 

 either directly or indirectly. Further, the response (of the ventricle) to 

 indirect stimulation (i.e. of the auricle) cannot be distinguished from the 

 beat evoked by the natural stimulus, arising at and propagated from the 

 venous orifices. The phenomena of the beat may therefore be studied in 

 the same order as those of a single muscular contraction. As, however, 

 we possess but little information as to the chemical and thermal changes 

 which in the heart are associated with activity, the present subject need 

 only be discussed here under the following headings : 



1. The mechanical phenomena of the response ; 



2. The electrical phenomena of the response ; and 



3. The physiological differences between cardiac excitability and 

 that of ordinary muscle. 



THE MECHANICAL RESPONSE. 



For reasons similar to those previously given in relation to skeletal 

 muscle, the systole of the ventricle of the frog's heart may be taken 

 to represent the normal action of cardiac muscle. The most striking 



