HISTOLOGY OF THE FROG 



95 



muscle-fibre is of elongate fusiform shape, thickest in the 

 middle, and pointed, more rarely forked, at the ends. The 

 bulk of the fibre is composed of a highly contractile, doubly- 

 refracting substance, which is not cytoplasm, but the product 

 of the cytoplasm of the cell from which the fibre was formed. 



Fig. 17. 



Group of unstriped muscle fibres from the bladder, n, the nuclei ; /, the 

 granular remains of the cell protoplasm ; _/J the longitudinally striated 

 contractile portion. 



The cellular character of each fibre is shown by the nucleus, 

 which is generally elongated and oval, and exhibits an intra- 

 nuclear network of chromatin. At each end of the nucleus are 

 a few granules, the remains of the cytoplasm from which the con- 

 tractile substance was formed. The plain fibres exhibit a faint 

 longitudinal striation, and each is contained in a very delicate 

 homogeneous sheath, which can only be seen when the fibre is 

 twisted or torn across. The individual fibres fit closely side by 

 side, and are gathered into larger or smaller bundles, which are 

 attached to the connective tissue layers of the visceral organs. 

 Striped muscular fibre (fig. 18, A) from a skeletal muscle 

 differs greatly in appearance from unstriped. The individual 

 fibres are generally gathered together into bundles or fasciculi, 

 and the bundles are united together to form the whole muscle. 

 The last-named is invested by a sheath of connective tissue, 

 called the perimysium, within which the bundles extend from 

 end to end of the muscle, being only partially separated from 

 one another by inward prolongations of the perimysium, the 

 so-called endomysium. The bundles may be large or small, 



