THE TISSUES OF MOTION 



79 



the use of a rigid and jointed skeleton, and the second is with such a 

 support. To understand how the first method can be used to produce 

 rapid and accurate motion, study the use of muscle in the arm of a squid 

 or octopus, or the proboscis of an elephant. The action of muscle with 

 a jointed skeleton, is doubtless already understood by the reader from 

 seeing figures of this operation in man and in a bivalve mollusk. As 

 the structure of muscle systems that operate without a skeleton is often 



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FIG. 79. Diagram of several ways in which muscle cells are arranged to form muscle tissue. 



a subject of histological study, we shall briefly examine an example of 

 it in a squid's arm. Figure 80 shows a line-drawing of a transection of 

 this organ, and the lettered bundles of muscle are used at this point as 

 follows. 



All muscles cut in cross section can be used as retractors to shorten 

 the arm. The most powerful retractors are seen to be in the outer part 

 of the mass. They are also used for the partial contractions that bend 

 the squid's arm. The method of extension is more difficult to see. 

 There being no rigid frame, the only method is to use some lateral 

 compressions which will force the mass to extend in the direction of the 

 arm's long axis and at right angles to the plane of the figure. 



An examination of these lateral muscle bundles will show that they 

 are confined to a circular, central core, sharply defined from the outer 

 layer of longitudinal fibers by a sheath of delicate connective tissue. 

 This core contains some small bundles of the longitudinal fibers, but the 

 greater part of its mass, outside of its non-contractile nervous ganglion 

 in the center, is occupied by fiber bundles that lie in all possible direc- 

 tions in the plane transverse to the long axis of the arm. When these 

 muscles contract, the central core is compressed laterally and conse- 

 quently extended longitudinally, while the outer tissues are, of course, 

 extended with it. 



