BOOK XI 



THE TISSUE OF MOTION 



CHAPTER LX 

 THE MECHANISM OF MOVEMENT 



A UNICELLULAR organism, such as the amoeba, moves by the flowing 

 of its protoplasm in one or other direction, the rest of the cell flowing 

 after the protrusion or pseudopodium. In other unicellular forms, 

 the development of one or more cilia or flagella enables the organism 

 to move, often with a relatively high rate of speed. In the multi- 

 cellular organizations this function of motion has been assigned to 

 special cells. Such cells are termed the muscle cells. The fully 

 specialized muscle cell contracts with a force, rapidity, and frequency, 

 far beyond the power of less specialized protoplasm. Its greater power 

 and efficiency have been acquired by the development within the 

 protoplasm of long, and exceedingly slender, contractile structures 

 the muscle fibrils lying parallel to the long axis of the cell and in 

 the direction of motion. Fibrils vary in the degree of differentiation. 

 Some exhibit a marked cross-striation, and are termed striated; others 

 are unstriated. The fibril affords the essential mechanism of rapid 

 motion. 



In the higher animals the principle of locomotion is that the moving 

 part first becomes angular in shape, and then straightens itself out 

 against some resisting substance ; the principle being the same whether 

 the organ of locomotion be fin, wing, or leg. Force exerted against 

 resisting water, air, or earth, and reacting in proportion to the resist- 

 ance, imparts movement to the body of the animal. 



The principle of the lever is applied in the various movements of 

 the body. There are three kinds or orders of levers (see Fig. 262). 

 (1) The first order, in Avhich the fulcrum (F) lies between the force 

 applied (P) and the resistance overcome (W), as exemplified in a pair 

 of scissors; (2) the second order, in which resistance (W) lies between 

 the fulcrum (F) and the force applied (P) e.g., in nutcrackers ; (3) the 

 third order, in which the force (P) lies between the fulcrum (F) and 

 the resistance ( W), as, for example, in a pair of sugar-tongs. By means 

 of levers the power applied may be augmented or the range and 

 rapidity of movement increased. In the body the power is usually 



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