ACTION OF THE MUSCLES. 345 



5. The penniform, or pen-shaped, which have their 

 fibres arranged obliquely on each side of the tendon, as 

 the rectus femoris, a muscle of the thigh. 



6. The semi-penniform, which have their fibres ar- 

 ranged on one side of the tendon, as the peron&us 

 longus, a muscle of the outside of the leg. 



7. The complicated, or compound, which have two, or 

 more tendons, as the flexors of the fingers ; or a variety 

 in the insertion of oblique fibres into the tendons, as the 

 linguales, muscles of the tongue. 



III. Action of the Muscles. 



1 . The voluntary, or those which are subject to the 

 will, as those of the legs and arms, or those of locomo- 

 tion. 



2. The involuntary, or those which act independently 

 of the will, as those of the heart. 



3. The mixed, or those which act imperceptibly, but 

 yet are subject, more or less, to the control of the will, as 

 the muscles of respiration. 



CONTRACTIBILITY OF THE MUSCLES. We have al- 

 ready given a definition of this property of the muscles ; 

 but, as the agent of motion in the animal system, and the 

 principal characteristic of these organs, some illustrations 

 of this property ought to be stated. 



Contractibility, constituting muscular action, consists 

 in the movement of the middle portions of the fibres to- 

 ward the two ends, where they are fixed. Thus, in the 

 amputation of a limb, as soon as the muscles are divided, 

 the two portions contract toward their fixed ends, leaving 

 a gap at the point of division, which is more or less wide, 

 according to the length and power of the muscle. Every 

 movable point in the solid animal frame is situated be- 

 tween two muscular powers, acting in opposition to each 

 other ; thus are produced flexion and extension, elevation 

 and depression, abduction and adduction, all by the dif- 

 ferent ways in which muscular contraction is exerted." 



What is a radiated muscle ? What a ventricose ? What a penniform ? 

 What a semi-penniform? What a complicated? What are the voluntary 

 muscles? What the involuntary? What the mixed? What does con- 

 tractibility consist in ? 



