FIG. 74. 



64 NORMAL HISTOLOGY. 



it will be seen that the lateral apposition of the thicker parts of the 

 contractile fibrilla produces the dark band, or transverse disk, while 

 the row of minute spherical masses appears as the interrupted dark 

 line bisecting the light zone, or intermediate disk. The threads 

 bridging between these beads and the chief 

 mass of the fibrillae are too delicate to be 

 appreciated under ordinary powers, and that 

 portion of the fibre corresponding to the 

 lateral disk consequently appears as if made 

 up of the lighter sarcoplasm alone. 



In certain forms of invertebrate muscle a 

 more complicated arrangement exists, since 

 on either side of the intermediate disk a row 

 of dark granules crosses the light lateral disk, 

 forming a dim secondary disk ; these gran- 

 ules are connected with the intermediate and 

 transverse disks by delicate bridges of con- 

 tractile substance, along which they occur as 

 local thickenings. The dim transverse disk 

 sometimes contains a central lighter band, 

 the median disk of Hensen, which is due, 

 probably, to diminished thickness of the con- 

 tractile fibrils. 



The contractile nbrillse, however, are not 

 uniformly distributed throughout the fibre, 

 but are aggregated into bundles the muscle- 

 columns each of which is enveloped in a thicker layer of the 

 sarcoplasm than the partitions separating the individual fibrillse. 



In transverse section each muscle-fibre presents a number of 

 small, polygonal, dark areas, enclosed by lighter lines, which areas, 

 under high amplification, exhibit minute punctations. These areas 

 are sections of the muscle-columns and correspond to Cohnheim's 

 fields, the dots being sections of the individual constituent fibrillae; 

 the lighter intervening and surrounding substance is the sarcoplasm, 

 thicker layers of which surround and separate the larger groups into 

 which the muscle-columns are further collected. 



The individual muscle-fibres, which usually are not circular in 

 cross-section, but rather irregularly polygonal with rounded angles, 

 are held together by a small amount of areolar tissue, the endo- 

 mysium. They are grouped into primary bundles, which latter 

 are enveloped and separated from other primary bundles by the 

 thicker bands of connective tissue constituting the perimysium. 

 The primary bundles are united to form larger secondary groups or 

 fasciculi, upon the width and arrangement of which the coarseness 



A, diagram of arrangement 

 of the contractile substance 

 according to the view of Rol- 

 lett : the granular figures rep- 

 resent the contractile element-;, 

 the intervening light areas the 

 sarcoplasm ; B, small muscle- 

 fibre of man ; the correspond- 

 ing parts in the two figures are 

 indicated : t, i, I, respectively 

 the transverse, intermediate, 

 and lateral disks; , muscle- 

 nuc'ei. 



