chap, ix.] STRIPED MUSCULAR TISSUE. 69 



vidual compartments. The total appearance pro- 

 duced is that of longitudinal striation, the sarcous ele- 

 ments of successive compartments forming fibrils 

 called the primitive fibrils. Sometimes in hardened 

 muscular fibres the substance of the sarcous elements 

 shows a middle transparent portion for the whole 

 contractile disc ; this appears to form a distinct 

 median transparency, known as the median disc of 

 Hensen. 



86. Of course, each such fibril is a successive row 

 of sarcous elements, with the corresponding portion of 

 Krause's membranes, and the adjacent portions of the 

 lateral discs. Generally, each fibril is thinnest at the 

 point of Krause's membrane and lateral discs, and 

 thicker at the part corresponding to the sarcous ele- 

 ments, so that in reality it is of a moniliform shape 

 (Haycraft). This varicose condition is the more appa- 

 rent the shorter and thicker the individual sarcous 

 elements are (Fig. 4 3 A, B and c). 



These differentiations due to structure alone are 

 sufficient to produce a transverse striation of the mus- 

 cular fibres; but it must be borne in mind that a fibre 

 when contracted or shrunk, even in the smallest degree, 

 would show a transverse striation due to the above- 

 mentioned undulating surface. Any other fibre with a 

 moniliform shape would show the same transverse 

 striation (Haycraft) ; and that usually observed on 

 hardened i.e., shrunk and more or less contracted 

 fibres, may be accounted for in this way. Fibres 

 stretched or prevented from shrinking generally show 

 pronounced longitudinal striation, but also very faint 

 cross striae ; these latter are due to the structural 

 differences. 



87. On observing a transverse section through a 

 fresh and living muscular fibre, the muscular substance 

 rnside the sarcolemma appears as a transparent ground- 

 glass-like substance, crossed here and there by bright 



