CHAPTER VIII 

 THE TISSUES OF MOTION 



MOTION is an almost universal attribute of protoplasm. Practically 

 all cells can move some part of their body, even if they have no means 

 of moving the body as a whole from place to place. Thus, the motion 

 may consist of internal operations, as circulation, ciliary movement, or 

 the many and varied acts of cell division. 



Or it may be exhibited as the contraction and relaxation of the cell- 

 body as a whole, as in some epithelial and other cells. Usually the act 

 of relaxation, as performed by elastic and other non-living parts of the 

 cell, is supplanted by another motion in a different plane, which acts as 

 a counter to the first movement, restoring the mass to its previous form 

 and shape. 



In some free cells, as Amoeba and the white blood corpuscles, these 

 movements result in complex flowing and creeping motions that move 

 the entire cell from place to place. In other unicellular forms the more 

 or less numerous cilia or flagella on the body, which is rigid, move the 

 creature in any direction and at high rates of speed. Both of these forms 

 of movement are found in the simply organized and small unicellular 

 animals, but in the great majority of higher form this function of moving 

 is all performed by special cells, only, of the organism, and these cells 

 present as high and complete a specialization as we find in nature. We 

 shall call them the muscle cells. The cytoplasm of muscle cells is of char- 

 acteristic granular structure and is called sarcoplasm. The granules 

 are known as myochondria. 



The fully specialized muscle cell can contract with a force, rapidity, 

 and quick succession far beyond the power of less specialized protoplasm. 

 It gets this greater power and efficiency from the development in its 

 cytoplasm of certain long thread-like regions of contractile substance, 

 the muscle fibrils. These fibrils are the cell-organs of contraction and 

 vary in appearance and number according to the needs of the tissue. 

 They are formed as plastid-areas of the sarcoplasm. 



They lie parallel to the long axis of the cell and in the direction of 

 motion. They vary in the completeness with which they are differen- 

 tiated out of the sarcoplasm of the muscle cell. They also differ in struc- 

 ture so that they may be classed as striated or non-straited fibrils. This 



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