88 THE WAVE OF CONTRACTION. [BOOK i. 



and the changes in the muscular substance started in the middle 

 of the muscular fibre travel thence to the two ends of the fibre. 

 In an ordinary skeletal 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 muscle 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 muscular 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 con- 

 traction 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 cur- 

 rent," 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 thicken- 

 ing 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 farther 

 lever. The contraction, started by the stimulus, in travelling along 



