120 THE HUMAN BODY. 



elbow joint. Where a muscle passes over an articulation 

 it is nearly always reduced to a narrow tendon; otherwise 

 the bulky bellies lying around the joints would make them 

 extremely clumsy and limit their mobility. 



Origin and Insertion of Muscles. Almost invariably 

 that part of the skeleton to which one end of a muscle io 

 fixed is more easily moved than the part on which it pulls 

 by its other tendon. The less movable attachment of a 

 muscle is called its origin, the more movable its insertion. 

 Taking for example the biceps of the arm, we find that 

 when the belly of the muscle contracts and pulls on its 

 upper and lower tendons, it commonly moves only the fore- 

 arm, bending the elbow-joint as shown in Fig. 49. The 



1 io. 40. The biceps muscle and the arm-bones, to illustrate how, under ordi- 

 nary circumstances, the elbow joint is flexed when the muscle contracts. 



shoulder is so much more firm that it serves as a fixed 

 point, and so that end is the origin of the muscle, and the 

 forearm attachment, P, the insertion. It is clear, how- 

 ever, that this distinction in the mobility of the points oi 

 fixation of the muscle is only relative for, by changing the 

 conditions, the insertion may become the stationary and 

 origin the moved point ; as for instance in going up a 

 rope "hand over hand." la that case the radial end of 

 the muscle is fixed and the shoulder is moved through 

 space by its contraction. 



Different Forms of Muscles. Many muscles of the 

 Body have the simple typical form of a belly tapering to a 



