62 HUMAN PHYSIOLOGY. 



bone to which they are attached around its own axis without pro- 

 ducing any. great change of position are known as rotators, and are 

 in association with the enarthrodial or ball-and-socket joints. Muscles 

 which impart an angular movement of the extremities to and from 

 the median line of the body are termed abductors and adductors. 



In addition to the actions of individual groups of muscles in 

 causing special movements in some regions, several groups of muscles 

 are coordinated for the accomplishment of certain definite functions 

 e. g., muscles of respiration, mastication, expression. The coordina- 

 tion of axial and appendicular muscles enables the individual . to 

 assume certain postures, such as standing and sitting ; to perform 

 various acts of locomotion, as walking, running, swimming, etc. 



Levers. The function or special mode of action of individual 

 muscles can be understood only when the bones with which they are 

 connected are regarded as levers whose fulcra or fixed points lie in 

 the joints where the movement takes 

 place, and when the muscles are consid- 

 ered as sources of power for imparting 

 " *T movement to the levers, with the object 



^ 1 of overcoming resistance or raising 



^yy P^ weights. 



^ In mechanics, levers of three kinds or 



(3) orders are recognized, according to the 



relative position of the fulcrum or axis 

 FIG. 5. THE^THREE ORDERS of motioilj the ap pHed power, and the 



weight to be moved. (See Fig. 5.) 



In levers of the first order the fulcrum, F, lies between the weight 

 or resistance, W, and the power of moving force, P. The distance 

 P-F is known as the power arm, the distance W-F as the weight 

 arm. As an example of this form of lever in the human body may 

 be mentioned : 



1. The elevation of the trunk from the flexed position. The axis 

 of movement, the fulcrum, lies in the hip-joint ; the weight, that 

 of the trunk, acting as if concentrated at its center of gravity, 

 lies between the shoulders ; the power, the contracting muscles 

 attached to the tuberosity of the ischium. The opposite move- 

 ment is equally one of the first 'order, but the relative positions 

 of P and W are reversed. 



2. The skull in its movements backward and forward upon the atlas. 

 In levers of the second order the weight lies between the power and 



