SPECIAL PHYSIOLOGY OF MUSCLES. 55 



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 

 adiuctors. 



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 coordination of axial and appendicular mus- 

 cles enables the individual to assume certain postures, such as standing and 

 sitting; to perform various acts of locomotion, as walking, running, swim- 

 ming, 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 re- 

 garded as levers whose fulcra or fixed points lie in 



the joints where the movement takes place, and 



when the muscles are considered as sources ^ ^ (l) 



of power for imparting movement to the levers, * A 



with the object of overcoming resistance or p ^ 



raising weights. A W P * 



In mechanics, levers of three kinds or orders 4 



are recognized, according to the relative posi- * (3) 



tion of the fulcrum or axis of motion, the applied * 



power, and the weight to be moved. (See FlG - 5- THE THRBK ORDERS 



Fig- 5-) 



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

 tance, 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 move- 

 ment, 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 movement 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 the fulcrum 

 As an illustration of this form of lever may be mentioned: 



i. The depression of the lower jaw, in which movement the fulcrum is the tem- 

 poromaxillary articulation; the resistance, the tension of the elevator 

 muscles; the power, the contraction of the depressor muscles. 



