MUSCULAR SYSTEM. 



MUSCLES. 



MUSCLES are structures attached to movable parts. 

 They are endowed with the property of contractility ; 

 that is, when thrown into action they shorten in the 

 direction of their fibres and draw together the parts to 

 which they are attached. This power of contraction is 

 due to the irritability of the muscle, and is exercised 

 within certain limitations as often as the muscle is stimu- 

 lated to act either normally through nerve impression or 

 through some exterior physical agency, as a blow. 

 Muscles are of two kinds, voluntary and involuntary. 

 The voluntary muscles are transversely striped and are 

 called striated. The involuntary have no transverse 

 markings. They are both made up of bundles of fibres. 

 The voluntary or striated muscles act under the influence 

 of the will ; they are composed of a number of delicate 

 prismoidal bundles, which run in the same direction. 

 These bundles are held together by the fibrous sheath 

 of the muscle or external perimysium; each bundle 

 is separately invested by the internal perimysium. A 

 bundle is made up of fibres, and each fibre invested by 

 an exceedingly delicate sheath of connective tissue, the 

 endomysium or sarcolemma. The fibres run for about an 

 inch, are about -g-^ inch in thickness, are flattened by 

 mutual pressure, and terminate either in a tendon or 

 another fibre continued in the same direction ; they do 

 not branch or inosculate except in the heart. Each fibre 

 is composed of extremely delicate fibrillae, ^-J^TT inch in 

 thickness, divided into a series of segments y-^-g-o inch 

 (172) 



