CHAPTER V. 



THE PHYSIOLOGY OF MOVEMENT. 



Of the four phenomena presented by an animal, that which more im- 

 mediately interests the physiologist is movement, for the reason that it is 

 not only the animal's most characteristic form of activity, and that which 

 serves to distinguish it in the main from forms of vegetable life, but its 

 solution affords an explanation of many physiologic processes occurring 

 within the human body. It is also for this reason that movement constitutes 

 for the most part the subject-matter of physiologic experimentation. 



The movements of the body may for convenience be divided into two 

 groups, viz., external and internal. 



The external movements are exhibited mainly by the head and extremities 

 and may be either special as when the animal changes the relation of one 

 part of the body to an other, or general as when it changes its position 

 relatively to the environment as in the various acts of locomotion. The 

 external movements are the result of the cooperation of the skeletal muscles 

 and the bones of the skeleton to which they are attached. The muscles 

 possess the power of suddenly shortening or contracting and by virtue of 

 their relation to the bones impart to them all the external movements char- 

 acteristic of the animal. The change of relation of the bones and hence of 

 the parts of the animal of which they form a part, are dependent on the 

 construction of the joints. 



In the execution of the movements the animal of necessity meets with 

 various forms of resistance, viz., gravity, cohesion, friction, etc., which tend 

 to oppose the movement. When its different parts are applied or directed, 

 either volitionally and in a determinate manner, or non-volitionally and in 

 an indeterminate or reflex manner, to the overcoming of these opposing 

 forces in the environment, the animal may be said to be doing work. 



In the animal as in the physical machine, work is accomplished by the 

 intermediation of levers. In the animal machine, the levers are found in 

 the bones of the skeleton and more particularly in the long bones of the 

 extremities, the fulcra of which, the points around which they move, lie in 

 the joints. 



That a lever may be effective as an instrument for the accomplishment of 

 work it must not only be capable of moving around its fulcrum, but it must 

 at the same time be acted on by two opposing forces, one passive, the other 

 active. In the movements of the bony levers of the animal body, the 

 passive forces to be overcome are largely those connected with the environ- 

 ment, e.g., gravity, cohesion, friction, etc., the active forces by which 

 these are opposed and overcome through the mediation of the bony levers, 

 are found in the muscles attached to them. The muscles are therefore to 

 be regarded as the seat of those active energies that impart movement to 

 the levers. 



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