138 STUDIES IN ADVANCED PHYSIOLOGY. 



It explains, finally, why the working body will eliminate 

 practically no more nitrogen from the kidneys than an idle 

 one. Much in the same way as a machine standing still 

 may have as much rust and wear and tear in its machinery 

 as an engine in proper use. 



The reason at last why in a working muscle the arteries 

 are thrown open and the arterial blood carrying the oxygen 

 and the nutritive substance of the plasma streams pell 

 mell through the muscle is, therefore, not to have it con- 

 tract the more, but to give it the opportunity of repair- 

 ing its substance as rapidly as it is being disintegrated. 

 The blood stream would be like the soldier carrying the 

 ammunition to the gunners, in order that they might make 

 good with new powder the losses resulting in their rapid 

 firing. 



The Mechanics of the Muscles. 



THREE CLASSES OF LEVERS. 



With the exception of a few muscles which surround 

 cavities, like the muscle around the mouth, all voluntary 

 muscles are inserted into bones, and produce with these 

 various kinds of levers used in supporting or moving the 

 frame. In every lever there are three points : the point about 

 which the lever turns, called the " fulcrum," the point at 

 which the weight is placed, and the point at which the 

 power is applied. According to the way in which these 

 three points are grouped, levers are divided into those of 

 the first class, the second class, and the third class. In a 

 lever of the first class the fulcrum lies between the "power" 

 and the "weight." A familiar illustration of such a lever 

 is found in the ordinary weighing balance, which is a beam 

 suspended in the middle from a fixed point, the fulcrum, 

 while the ' ' weight ' ' and the ' ' power ' ' are applied at the 

 opposite ends. Numerous other illustrations might be ad- 

 duced. The further illustration of the teetering-board will 

 suffice. There are few levers of this kind in the body. 

 The nodding of the head is one. In this instance the ful- 

 crum is at the point where the occipital bone rests on the 



