420 ELECTRO-PHYSIOLOGY CHAP. 



muscles of the limbs, on applying electrical excitation to the former 

 directly (by puncturing the spinal cord, or excising one end), to the 

 latter from the nerve-trunk. Since the tortoise can withdraw its 

 head with tolerable rapidity, and almost twitches its legs, at least 

 when its nerves are artificially excited by induction shocks, its failure 

 of secondary response both to single twitches and to tetanus is 

 very remarkable. The extent to which secondary excitation from 

 muscle to nerve (frog) is dependent on the velocity of the excita- 

 tion (contraction) wave is elegantly shown in the heart. While 

 Klihne (I.e.) obtained only weak secondary twitches from the 

 ventricle of the beating tortoise heart, which disappeared soon 

 after exciting it, i.e. long before any perceptible diminution of 

 pulse, the smaller but more rapidly beating frog's heart responds 

 much better, and the still more rapidly pulsating mammalian 

 heart notably gives vigorous secondary contractions. On the 

 other hand, Kiihne obtained, on exciting the nerve, as good 

 secondary twitches and secondary tetanus from the red gastroc- 

 nemius, as from the pale muscles of the rabbit, although its 

 twitch is essentially more sluggish. 



J. v. Uexktill (52) found that with uniform conditions, an 

 important factor in the results of secondary excitation was the 

 point at which the primary muscle (non-curarised sartorius) was 

 excited. " The simultaneous excitation of muscle-substance and 

 nerve transversely to the entrance of the latter into the sartorius 

 produces no secondary reaction, while pure muscular excitation, 

 like pure nerve excitation, results in secondary action under the 

 same conditions." Uexkiill showed by experiments on the 

 gracilis muscle that the occurrence of secondary effects is associ- 

 ated with the coexcitation of the nerve-endings. Under certain 

 presumptions (i.e. a latent period for the propagation of excitation 

 from the nerve end-organ to the muscle, and secondary activity 

 of the summit only of the variation curve of the action current) 

 the phenomenon might be explained as one of interference. 

 Uexkiill formulates the process as follows : " A stimulus reaches 

 the nerve end-organ and muscle-fibre simultaneously, it discharges 

 a wave of action in the latter which would throw the secondary 

 limb into excitation, were it not that the simultaneously excited 

 end-organ of the nerve discharged itself a moment later upon the 

 muscle. Hence instead of a simple action wave there are two 

 waves coupled together. These waves are inept for secondary 



