8 4 



HISTOLOGY 



These bundles take up the greater part of the fiber, and the sarco- 

 plasm lies between the groups of fibrillae, into which the fiber bundles 

 are divided. This grouping of the fibrillae is clearly seen in the cross 

 sections of sucker muscle. The fibrillae are grouped, near the periphery 

 of the fiber, into plates of one fibril in thickness. The form of the 

 plates is due to the presence of a mass of heavier cytoplasm that sur- 

 rounds and binds them together. This is the cement substance. 



These peripheral plates extend for the same distance into each fiber, 

 whether it be large or small, and inside of that the fibrils are no longer 



FIG. 82. Transection of several related fibers of muscle in the sucker Catostomus. /., smallest 



fiber. 



a part of the peripheral plates, but are either placed singly or grouped 

 into smaller independent plates. Thus in the smallest fibers (as at Fig. 

 82, /.), the plates extend to the center, while all larger fibers have the 

 center made up of detached groups. It seems that the fibrils are thus 

 placed in thin layers side by side, in order that each and every fibril may 

 be in direct contact with some part of the sarcoplasm from which it 

 draws its food and stimulus, and on which it unloads its refuse materials. 



That the plates do not extend in a complete condition further inward 

 than they do is probably due to the requirements of the movement of 

 the fiber which does not contract and expand all its parts in perfect 

 unison, as will be shown elsewhere. 



The individual fibrils which show so distinctly in the longitudinal 

 sections cannot be ordinarily seen in a transverse section of the fiber. 

 This may be due to the optical properties of the cement substance which 

 surrounds each fibril and which binds them together in the plates. The 



