Reflexes, etc., of Mammalian Nerve-muscle. 251 



tension of the reflex contraction is attained later than that of the twitch. 

 (4) The crest of the reflex myogram tends commonly to be more plateau-like 

 than is that of the twitch.* (5) The decline of tension of the reflex contrac- 

 tion is more prolonged than is that of the twitch. (6) The reflex myogram 

 elicited by the break-shock vid each of the afferent nerves iised is of more or 

 less characteristic form for each of those nerves severally. Thus, internal 

 saphenous gives a lower and more prolonged ciest-plateau (figs. 2, .3) than 

 does popliteal. 



These differences observable between break-shock reflex contraction and the 

 maximal twitch evoked from motor nerve suggest that, while in the latter the 

 motor nerve conveys but a single nervous impulse per fibre, in the former the 

 centrifugal nervous impulses are, at least in some of the efferent fibres, 

 repetitive. It was shown by Sassa and myself (2) that this difference persists 

 after the proprioceptive reflex arc of the reacting muscle itself is broken. 

 The responsible difference, therefore, between the nervous preparations com- 

 pared for reflex and twitch is that while that of the latter consists of motor 

 nerve, muscle, and neuromyal junction, that of the former comprises in 

 addition the reflex (spinal) centre and the central stump of the afferent 

 nerve. The added factor that at once appeals as that most probably 

 responsible for the difference between the reactions of the two preparations 

 is doubtless the spinal centre present in the former and absent from the 

 latter. Prior, however, to accepting this inference, certain contingencies 

 have to be borne in mind. 



In the mammalian motor nerve-muscle preparation, as in that of the frog, 

 the muscle response under progressively increasing break-shock stimuli, 

 starting from threshold value upwards, is a twitch contraction, which 

 progressively increases in strength through a certain range of stimulus 

 increments (sub-maximal), and then remains practically without further 

 increase (fig. 6), i.e., is " maximal " throughout a very considerable succeeding 

 range of further increments of stimulus. When, however, this long range of 

 maximal stimuli is followed to its stronger end, there comes a point where, 

 with further increase of the break-shock, the muscular response begins again 

 to increase, and increases rapidly (fig. 6). This point in the inductorium 

 used in these experiments showed considerable variation, but was usually 

 where secondary coil was about 15 cm. on the scale from primary. 



* In view of the precautions taken to minimise lag and overthrow in the myograph 

 used in these records, it is noteworthy that the crest of the motor twitch contraction in 

 this mammalian muscle with intact blood circulation appears in the myograms not as a 

 round-curved peak, but rather as a plateau, although the plateau is short, ending slightly 

 more abruptly than it began. 



VOL. XCII. — B. U 



