8o 



HISTOLOGY 



\ 



The extensory core is placed at the most efficient point in the section, 

 at the center. Not that the outer edge would not serve as an extensor 

 just as well, but because this outer edge must be reserved for the bend- 

 ing muscles, which serve to bend, wave, and curl the pliable arms in all 

 directions. They can do this, too, independently of the state of exten- 

 sion or retraction of the arm, owing to their independence of the core. 

 Watch a live octopus or squid and see that its arm can twist and 



curl, extend, or con- 

 tract in practically 

 any direction. All 

 these movements are 

 due to contractions 

 of muscle cells, for 

 no muscle cell can 

 extend itself. This 

 must be done for 

 it by some other 

 counteracting 

 muscle cell properly 

 placed to oppose it. 

 Study the action 

 of the muscle layers 

 in the intestine of 

 vertebrates also. 

 For the origin of the 

 muscle tissues, see 

 later in this chapter. 

 Several features 

 have been used to 

 classify muscle cells, 

 but no two of them 

 agree except within 

 very narrow limits. 



Their control by the will divides them into voluntary and involuntary 

 muscle cells, the markings of the fibrillse serve to classify them as 

 striated and non-striated muscle cells, the number of their nuclei 

 makes them mono- or multinuclear, and less important features group 

 them into incomplete classes, as epithelial, branched, and circular 

 muscle cells. Our study may well begin with the cylindrical, multinu- 

 cleated, striated fiber. Such a muscle cell is universally found in 

 forms of all grades where efficiency and economy is needed. It is of 

 high specialization and is under control of the will. The example 

 taken is from the common brook sucker, Catostomus communis. 



FIG. 80. Transaction of squid's arm; r,, retractors; ex., ex- 

 tensors; />., flexing retractors; m., membrane separating the 

 extensor core (with its retractors) from flexors or bending mus- 

 cles; n., central nerve cord. X 80. 



