80 THE CONTRACTILE TISSUES. 



insisted that the relaxation is an essential part of the whole act ; indeed, 

 in a certain sense, as essential as the shortening itself. 



54. Optical changes in a muscular fibre during contraction. So far we 

 have been dealing with the muscle as a whole and as observed with the 

 naked eye, though we have incidentally spoken of fibres. We have now, 

 confining our attention exclusively to skeletal muscles, to consider what mi- 

 croscopic changes take place during a contraction, what are the relations of 

 the histological features of the muscle fibre to the act of contraction. Un- 

 fortunately, our knowledge of the minute structure of the fibre is as yet so 

 limited that any statements must of necessity be but speculative. When 

 muscle contracts there is a translocatiou of molecules whereby there occurs 

 not only a change of form, but other optical (polariscopical and microscopi- 

 cal) alterations which are due to the movement of refractive particles. 



The long cylindrical sheath of sarcolemma is occupied by muscle sub- 

 stance. After death the muscle substance may separate from the sarco- 

 lemma, leaving the latter as a distinct sheath, but during life the muscle 

 substance is adherent to the sarcolemma, so that no line of separation between 

 the two can be made out ; the movements of the one follow exactly all the 

 movements of the other. 



Scattered in the muscle substance, but, in the mammal, lying for the most 

 part close under the sarcolemma, are a number of nuclei, oval in shape, with 

 their long axes parallel to the length of the fibre. Around each nucleus is 

 a thin layer of granular-looking substance very similar in appearance to 

 that forming the body of a white blood-corpuscle, and like that often spoken 

 of as undifferentiated protoplasm. A small quantity of the same granular 

 substance is prolonged for some distance, as a narrow conical streak from 

 each end of the nucleus, along the length of the fibre. 



With the exception of these nuclei with their granular-looking bed and 

 the end-plate or end-plates, to be presently described, all the rest of the 

 space enclosed by the sarcolemma from one end of the fibre to the other 

 appears to be occupied by a peculiar material, striated muscle substance. 



It is called striated because it is marked out, and that along the whole 

 length of the fibre, by transverse bands [Fig. 28], stretching right across 

 the fibre, of substance which is very transparent, bright 

 [FIG. 28. substance, alternating with similar bands of substance 



which has a dim cloudy appearance, dim substance ; 

 that is to say, the fibre is marked out along its whole 

 length by alternate bright bands and dim bands. The 



^ t bright bands are on an average about 1 p. or 1.5 >j. and 



Diagrammatic Repre- the dim bands about 2.5 // or 3 /JL thick. By careful 

 sentation of a Muscle- focusing, both bright bands and dim bands may be 

 ^Tri-ht'bSsr^ 85 traced through the whole thickness of the fibre, so' that 

 the whole fibre appears to be composed of bright discs 

 and dim discs placed alternately one upon the other along the whole length 

 of the fibre, the arrangement being broken by the end-plate and here and 

 there by the nuclei. 



55. We may now return to the question, What happens when a con- 

 traction wave sweeps over the fibre ? 



Muscular fibres may be examined even under high powers of the micro- 

 scope while they are yet living and contractile ; the contraction itself may 

 be seen, but the rate at which the wave travels is too rapid to permit satis- 

 factory observations to be made as to the minute changes which accompany 

 the contraction. It frequently happens however that when living muscle 

 has been treated with certain reagents, as for instance with osmic acid vapor, 

 and subsequently prepared for examination, fibres are found in which a 



