MUSCULAR AND SKELETAL SYSTEMS. 2 2I 



of parallel fibers, and surrounds a hollow organ without 

 skeletal attachment. 



A muscular fiber is apparently evenly cylindrical, 

 without any evidence of cellular origin. But in embry- 

 onic development it is seen to be formed by a coales- 

 cence of elongated nucleated cells. This is well seen 

 in the fibers of the embryonic heart of the monkey 

 (Fig. 141). 



Voluntary Muscle. — Form. — The typical form of a 

 voluntary muscle is seen in the muscles of the limbs — 

 e.g., the biceps — which is shown in Fig. 146, page 228. 

 It is attached to the skeleton at both ends. The nearer 

 and more fixed point is called the origin, the farther and 

 more movable point, the insertion. Between the two 

 the largest and most contractile part is called the belly. 

 The fibers all unite to form the tendon, by which it is 

 attached to the skeleton. This is the type, but there is 

 considerable variation. Sometimes the fibers are con- 

 vergent. This is mainly in muscles connecting the limbs 

 with the trunk, as in the deltoid, the pectoral, etc. 

 Sometimes the fibers are nearly parallel, as in the mas- 

 seters. 



Structure.— A voluntary muscle is a definite mass in- 

 vested by a thin fibrous membrane — sheath. If cut into we 

 find it made up of bundles of fibers — fasciculi (Fig. 142, A). 

 These are conspicuous in cured meat, such as corned 

 beef. They constitute the grain of the flesh. These, 

 too, are invested with a thin membrane of fibrous tissue. 

 These bundles are in their turn composed of fibers lying 

 parallel to one another in the bundle. Each fiber is 

 also invested with a very thin sheath of fibrous tissue. 

 The fibers themselves are supposed by some to be com- 

 posed of smaller fibrillse, but this is doubtful. Fig. 142, B, 

 represents a single fiber, broken and twisted, showing 

 the sheath. We may regard the whole muscle as pene- 



