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 or Skele to -muscle Movements. The external move- 

 ments 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 

 another, or general, as when it changes its position relatively to the en- 

 vironment as in the various acts of locomotion. The external movements 

 are the result of the cooperation of the bones of the skeleton and the muscles 

 which are attached to them. 



The skeleton, though very mobile, imparts nevertheless, a certain 

 degree of rigidity and fixity to the body; were it not for this the body would 

 be but a shapeless mass and incapable of performing any of its characteristic 

 external movements. The various movements of the individual limbs 

 or of the entire body are made possible by the presence of joints. 



Without the presence of joints such as are found in the vertebral 

 column and in the limbs, external movements would be impossible. 



The muscles impart movements to various parts of the body. This 

 they do by suddenly shortening and widening whereby their extremities 

 are approximated. The majority of the muscles of the body are attached 

 to the bones in such a manner that when their form is altered, they change 

 not only the relation of the bones with reference to each qther, but perhaps 

 also the individual's relation to surrounding objects. The muscles thus 

 become the active organs in both motion and locomotion, in contradis- 

 tinction to the bones which may be regarded as the passive organs in the 

 performance of the corresponding movements. 



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. 



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