230 PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



objected) that j must be taken as the fixed point or 

 fulcrum, and therefore we must treat the question as 

 falling under the second order of levers — i. e., the power 

 pulling upon the lever I, J = 7 inches, and the weight 

 pushing down on the lever 2, 3 = 6 inches, then we must 

 remember that the muscle pulls the body downward, 

 adding to the weight exactly as much as it pulls the 

 heel upward. Therefore, under this conception, we must 

 put the proportion thus — 7 : 6 :: 300 + x : x. yx = 6x-\- 

 1,800 — or x = 1,800. 



It has been determined by experiment that one square 

 inch of muscle will contract with a power equal to 

 about one hundred to one hundred and twenty pounds. 

 Why, then, does not the muscle break ? For dead 

 muscle can stand no such tensile strain as this. The 

 answer is : It would break if it were passive, and the 

 force was external to itself, but it is the attraction be- 

 tween the molecules of the muscle itself that develops 

 the pull ; attraction can not produce separation. Muscles 

 do break sometimes, but always from irregular contrac- 

 tion — i. e., one part contracts while another part does 

 not, and is therefore subject to tensile strain, just as the 

 tendon is. 



2. Locomotion. — In limb motion the origin or body 

 end of the muscle is fixed, and the insertion or limb end 

 moves; but these are interchangeable. If we fix the 

 limb, then the body moves. Thus, for example, if we 

 hold up the hands above the head and bring into action 

 the great muscles about the armpit, and also the biceps, 

 the elbow is brought down to the side and the fist to 

 the chin ; but if we fix the hands by taking hold of a 

 bar and bring into action the same muscles, the body 

 rises until the chin goes over the bar. Now locomotion 

 is nothing more than limb motion reacting against the 

 ground in walking, running, leaping, against the water 



