THE CONNECTIVE TISSUES. 57 



viscera and affording support for the epithelial, muscle, and nerve 

 tissues. 



THE ANIMAL BODY AS A MACHINE FOR DOING 



WORK. 



The animal body is characterized by the power of executing a 

 great variety of movements, all of which have reference to a change 

 of relation of one part of the body to another, or to a change of posi- 

 tion relatively to the environment, as in the various acts of locomotion. 

 Since in the execution of these movements the different parts are 

 of necessity applied or directed to the overcoming of opposing 

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

 the conception of the animal body as a machine for the accomplish- 

 ment of work the skeleton, the muscle and nerve tissues constitute the 

 three primary mechanisms, all of which bear certain definite relations 

 one to another. 



