ANIMAL AND VEGETABLE 99 



he employed was to count the number of electrical varia- 

 tions which accompany a voluntary contraction, on the 

 assumption that each fundamental unit of the contraction 

 has an electrical change as its concomitant. . . . The 

 number of electrical variations is found to be a fixed one 

 for each muscle, but to vary in different muscles. Various 

 spinal and cranial motor centres have thus different 

 rhythms, and of those hitherto studied the cells of the 

 motor fibres of the fifth cranial nerve have the highest 

 rate of discharge, 86 to 100 per second. In muscles 

 supplied by spinal nerves the rate is lower, 40 to 60." 



Many other proofs could no doubt be cited, but we 

 have an example of, as I think, variation of capacity in 

 Purkinje's fibres in the auriculo-venticular-bundle of cardiac 

 muscle. These are large, quadrangular cells with granular 

 protoplasm, and striated, it is said, only on the margins. 

 The slow rate of propagation of the wave suggests greater 

 capacity than in ordinary striated muscle, and therefore 

 either (1) the plates are closer together, (2) they are larger, 

 or, (3), what is more probable and indeed indicated by 

 physiological diagrams, they are connected in parallel. If 

 this is so the argument should apply with even greater 

 force to plain muscle, but, unfortunately, the structure of 

 the latter is not sufficiently defined to enable a definite 

 opinion to be given. 



In cardiac muscle the movement is rhythmical, and it 

 differs from that of voluntary and plain muscle in that, 

 subject to regular periods of rest, it is constant, whereas 

 in the others it is intermittent. We can readily under- 

 stand this when we remember that discharge or neutralisa- 

 tion does not take place instantaneously unless there is 

 actual contact. Regular periods of time or rest would be 

 necessary in any such circuit if it was required to work 

 continuously and automatically. The retardative action 

 is equally pronounced in the discharge as in the charge, and 



