82 THE HUMAN BODY 



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 is fixed is more easily 



moved than the part on which it pulls by its other tendon. The 

 ABC 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 forearm, bending the elbow-joint 

 as shown in Fig. 39. The 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 fore- 



grams ' illustrating arm attachment, P, the insertion. It is clear, 

 however, that this distinction in the mobility 



two terminal ten- o f tne po ints of fixation of the muscle is only rela- 



dons. o, a penm- 



form muscle ;c, a bi- tive, for, by changing the conditions, the m- 



penniform muscle. ,. , ,, ,. , ,-, 



sertion may become the stationary and the 

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

 "hand over hand." In 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 single tendon at 

 each end as A , Fig. 40, but others divide at one end and are called 

 two-headed or biceps muscles; while some are even three-headed or 

 triceps muscles. On the other hand, some muscles have no tendon 

 at all at one end, the belly running quite up to the point of attach- 

 ment; and some have no tendon at either end. In many muscles 

 a tendon runs along one side and the fibers of the 

 belly are attached obliquely to it: such muscles 

 (B, Fig. 40) are called penniform or featherlike; or 

 a tendon runs obliquely down the middle of the 

 muscle and has the fibers of the belly fixed ob- FIG. 41. A di- 

 liquely on each side of it (C, Fig. 40), forming a gas 

 bipenniform muscle: or even two tendons may run down the belly 

 and so form a tripenniform muscle. In a few cases a tendon is 

 found in the middle of the belly as well as at each end of it; 

 such muscles are called digastric. A muscle of this form (Fig. 41) 



c 



