78 THE CONTRACTILE TISSUES. 



tal muscle, however, as we have said, the fibres and bundles of fibres begin 

 and end at different distances from the ends of the muscle, and the nerve or 

 nerves going to the muscle divide and spread out in the muscle in such a 

 way that the end-plates, in which the individual fibres of the nerve end, are 

 distributed widely over the muscle at very different distances from the ends 

 of the muscle. Hence, if we suppose a single nervous impulse, such as that 

 generated by a single induction-shock, or a series of such impulses to be 

 started at the same time at some part of the trunk of the nerve in each of 

 the fibres of the nerve going to the muscle, these impulses will reach very 

 different parts of the muscle at about the same time and the contractions 

 which they set going will begin, so to speak, nearly all over the whole mus- 

 cle at the same time, and will not all start in any particular zone or area of 

 the muscle. 



53. The wave of contraction. We have seen, however, that under the 

 influence of urari the nerve fibre is unable to excite contractions in a mus- 

 cular fibre, although the irritability of the muscular fibre itself is retained. 

 Hence in a muscle poisoned by urari the contraction begins at that part of 

 the muscular substance which is first affected by the stimulus, and we may 

 start a contraction in what part of the muscle we please by properly placing 

 the electrodes. 



Some muscles, such, for instance, as the sartorius of the frog, though of 

 some length, are composed of fibres which run parallel to each other from 

 one end of the muscle to the other. If such a muscle be poisoned with 

 urari so as to eliminate the action of the nerves and stimulated at one end 

 (an induction-shock sent through a pair of electrodes placed at some little 

 distance apart from each other at the end of the muscle may be employed, 

 but better results are obtained if a mode of stimulation, of which we shall 

 have to speak presently, viz. the application of the " constant current," be 

 adopted), the contraction which ensues starts from the end stimulated, and 

 travels thence along the muscle. If two levers be made to rest on, or be 

 suspended from, two parts of such a muscle placed horizontally, the parts 

 being at a known distance from each other and from the part stimulated, 

 the progress of the contraction may be studied. 



The movements of the levers indicate in this case the thickening of the 

 fibres which is taking place at the parts on which the levers rest or to which 

 they are attached ; and if we take a graphic record of these movements, 

 bringing the two levers to mark one immediately below the other, we shall 

 find that the lever nearer the part stimulated begins to move earlier, reaches 

 its maximum earlier, and returns to rest earlier than does the further lever. 

 The contraction, started by the stimulus, in travelling along the muscle from 

 the part stimulated reaches the nearer lever some little time before it reaches 

 the further lever, and has passed by the nearer lever some little time before 

 it has passed by the further lever ; and the further apart the two levers are 

 the greater will be the difference in time between their movements. In other 

 words, the contraction travels along the muscle in the form of a wave, each 

 part of the muscle in succession from the end stimulated swelling out and 

 shortening as the contraction reaches it, and then returning to its original 

 state. And what is true of the collection of parallel fibres which we call 

 the muscle is also true of each fibre, for the swelling at any part of the 

 muscle is only the sum of the swelling of the individual fibres ; and if 

 we were able to take a single long fibre and stimulate it at one end, we 

 should be able, under the microscope, to see a swelling or bulging accom- 

 panied by a corresponding shortening, i. e., to see a contraction, sweep 

 along the fibre from end to end. 



If, in the graphic record of the two levers just mentioned, we count the 



