308 MUSCULAR CONTRACTION. L.ECT. XVI. 



sides always contains the point of support, or both of them, 

 according to the medium in which the animal moves. Air, 

 water, and the ground, offer resistance to the parts of an 

 animal which strike against, or press, upon, these media, 

 and the movement is produced in the same way that the 

 progression of steamboats and of locomotives is, effected, 

 by the impulse of the paddles against the water, and of the 

 friction of the wheels upon the rails. 



