CHAP. IX.] THE CONTRACTILE TISSUES. 313 



about to be described, are found to contain doubly refracting, positive, 

 imiaxal particles scattered through their substance. 



Structure of voluntary muscle. 



The second kind of muscular tissue is commonly known as volun- 

 tary, and transversely striated. It consists of elements or fibres, which 

 are exceedingly large when compared with fibre-cells, being about -5^0 th 

 of an inch in diameter (10 to 80 /A 1 ), and as much as from 1 to 1 J inches 

 long 2 . Each fibre is enclosed in a structureless elastic sheath or sarco- 

 lemma, rounded at its extremities, which either become attached to 

 tendons or aponeuroses, or lie overlapped by neighbouring fibres. 

 The contents of the sarcolemma when examined in a perfectly fresh 

 condition, as they may be in the case of cold-blooded animals, are of 

 a pale grey translucent appearance. They exhibit a very regular 

 series of transverse markings, but hardly a trace of longitudinal 

 striation, if care have been exercised in the preparation. The 

 transverse striation is due to an alternation of dim and bright lines 

 which commonly run continuously across the long axis of the fibre, 

 but which are sometimes interrupted by 'faults,' (to use a geological 

 term,) as if one portion of the fibre had slipped to a lower level than 

 the rest. The striae in the frog's muscle are exceedingly fine 

 and somewhat confusing. If we examine in the normal condition 

 the muscles of animals lower in the scale, we find the corresponding 

 elements both larger and more complex. This examination may 

 be made with very little preparation in the case of the limb-muscles 

 of Hydrophilus, fragments of which may be snipped out and mounted 

 without any addition, after the chitinous covering of the thigh has 

 been split, while in the case of Cyclops no preparation whatever is 

 needed other than fixing the specimen beneath a covering glass 3 . A 

 muscle of small diameter and at rest should be selected for observa- 

 tion. In such a specimen the most striking feature will still be the 

 alternation of darker and lighter bands. But the dark, or, more 

 strictly speaking, the dim band will be found more or less marked by 

 longitudinal lines, and to be traversed by a zone or region less 

 cloudy than the rest, to which the name of Hensens disc is given. 

 The lighter stripe, in its turn, is still more clearly divided by a thin 

 dark line called Krauses membrane, which under a sufficiently 

 high power in hardened specimens appears as a series often as 

 a double series of dots 4 . If such a muscular fibre were seen in 

 cross section, and in a perfectly normal state, it would present 

 the appearance of a homogeneous clear substance, thickly and evenly 



1 Eanvier, Trait'e technique d'Histologie, p. 468. 



2 Quain's Anatomy, eighth ed., Vol. n. p. 115. 



8 As was demonstrated to the author by Mr Marcus Hartog, in Cyclops the structure 

 of striated muscle, and the end-organs of the nerves in muscle, may be perfectly studied 

 in the Jiving, uninjured animal. 



4 This line is said to have been first seen by Dobie (Ann. of Nat. Hist., 2nd Ser.. 

 1849, Vol. m. p. 109). 



