A STUDY OF THE MUSCLES 197 



region. One muscle is especially developed in the back of 

 the calf of the leg, and is attached to the heel bone by the 

 tendon of Achilles (so called because in Greek mythology the 

 hero Achilles is said to have met his death by an arrow that 

 pierced this tendon). When the extensor muscle we have 

 been describing contracts, it raises the body on tiptoe (see 

 Fig. 89). The corresponding flexor, that causes one to stand 

 on the heels, runs from the knee down the front of the 

 tibia, its insertion being on the ankle bones. 



In the ventral wall of the abdomen, in the region of the 

 face, and in some other parts of the body, the muscles are 

 arranged in broad, flattened masses and serve as a movable 

 wall to inclose the cavities within. Like the muscles of the 

 appendages, they are attached to the skeleton or to each 

 other by tendons. 



Structure of Voluntary Muscle. A thick piece of steak 

 cut from a leg of beef furnishes good material for the study 

 of voluntary muscle. 1 One 

 sees that the muscle can be 

 separated into rather large, 

 more or less cylindrical or 

 prism-shaped masses, that 

 run along the leg bone. 

 Each of these masses is 

 called a muscle bundle. It 

 is surrounded by a tough, 

 glistening sheet of con- 

 TiPPtivp ti^np Pallprl wr FlG ' ^- Muscle Bundles (/) bound 



L P er ~ together to make a Piece of Muscle. 

 i-my'si-um (Greek peri = 



around + mys = muscle). When this perimysium is pulled 

 off, the bundle is found to be composed of smaller bundles, 

 and each of these is enveloped in a thin sheath of peri- 

 mysium. These smaller bundles, in turn, can be separated 

 still further until one gets a bundle, or piece of bundle, so 

 small that it can hardly be seen with the naked eye. 

 1 See "Laboratory Exercises," No. 34. 



