CHAP. IL] 



THE CONTRACTILE TISSUES. 



55 



dition in the living body, it is probably somewhat greater, and the 

 wave also probably travels with undiminished velocity and vigour 

 to the end of the fibre. In general, the velocity with which the 

 contraction wave travels, like the duration and character of the con- 

 traction, varies under different circumstances, being much influenced 

 by temperature, by the action of drugs, and especially by those 

 complex intrinsic changes which we speak of as fatigue or ex- 

 haustion. 



Seeing that the extreme limit of the length of a muscular fibre 

 is about 30 or 40 mm., it is evident that even when the stimulation 

 begins at one end and the wave travels at the more rapid rate, 

 the whole fibre is not only in a state of contraction at the same 

 time, but almost in the same phase of the contraction wave. In 

 an ordinary contraction occurring in the living body the stimulus 

 is never applied to one end of the fibre; the nervous impulse 

 which in such cases acts as the stimulus to the muscle, falls into 

 the fibre at about its middle, where the nerve ends in an end-plate, 

 and the contraction wave starting from the end-plate travels along 

 the muscular fibre in both directions. In such a case therefore, 

 still more even than in the urarised muscle stimulated artificially 

 at one end, must the whole fibre be occupied at the same time by 

 the wave of contraction. 



Changes in microscopic structure. When portions of living 

 irritable muscle are examined under the microscope, contraction 

 waves similar to those just described, but feebler and of shorter 



FlO. 10. MUSCULAB FIBRE UNDERGOING CONTRACTION. 



The muscle is that of Telephones melanurus treated with osmic acid. The fibre 

 at c is at rest, at a the contraction begins, at 6 it has reached its maximum. The 

 right-hand side of the figure shews the same fibre as seen in polarized light. (After 

 Engelmann. ) 



