VOLUNTARY MUSCLES 



Radius 



Ulna 



Tendon 

 Humerus 



are made up, as we have seen, of slender fibres having 

 a characteristic cross-striped, or striated, appearance. 

 When they contract, their fibres become shorter and 

 thicker, thus shortening and thickening the muscle as a 

 whole. The time required for contraction may be very 

 slow or as quick as one-tenth to one-twentieth of a sec- 

 ond. All muscles con- 

 nected directly or indi- 

 rectly with the bones 

 are classed as voluntary. 

 Involuntary muscles. 

 The muscles in the 

 Avails of the stomach, in- 

 testines and blood-ves- 

 sels are independent of 

 the control of the will 

 and are therefore called 



involuntary. They dif- FIG. 39. Contraction of biceps to flex 



fer from the voluntary 



in that they are comparatively short, are non-striated 

 or "smooth," and require as long as from ten to sixty 

 seconds for a complete contraction. 



Comparison of voluntary and involuntary muscles, 

 The distinction between the voluntary and the invol- 

 untary muscles cannot be drawn too closely, since, al- 

 though they differ markedly from each other in size 

 and structure, yet the voluntary muscles may act invol- 

 untarily. A good illustration of this is seen in the 

 movements due to sudden danger, as in winking or 

 starting at a noise. Although the muscles controlling 

 these movements are usually under voluntary control, 

 yet in times of danger they may act so quickly for the 

 protection of the body that interference by the will is 

 impossible. This is also true of many other muscles, 



