THE MUSCULAR SYSTEM 



55 



Structure. — Like all other organs, muscles are composed of 

 several types of tissue. White fibrous tissue forms a covering, 

 called the perimysium, for the muscle. On close examination 

 the muscle is found to be composed of numerous bundles of fibers 

 of muscular tissue. Each bundle is in 

 turn made up of smaller bundles, which 

 are composed of separate fibers. These 

 extremely minute fibers are similar to a 

 cylindric thread and are about 2 inches 

 long and J 50 o inch thick. The bundles 

 have a delicate sheath or covering, the 

 sarcolemma. Under the microscope fine 

 striations are seen at right angles to 

 the long axis of the fibers, hence the 

 name striated or . striped (Fig. 11). 

 When a muscle is subject to much strain, 

 tendinous fibers are mixed with the 

 fleshy fibers. 



Attachments.— These may be direct 

 to other muscles or to cartilages or 

 bones, or indirect by means of tendons. 

 Tendons are rounded or flattened cords 

 fitted to the fleshy ends of muscles. 

 The attachments determine the direc- 

 tion, extent, relation, and, in part, the 

 uses of muscles. The point of attach- 

 ment which usually remains fixed is 

 termed the origin; the point that is dis- 

 placed when the muscle contracts is the 

 insertion. Tendons are usually less 

 extensive than the fleshy part of the 

 muscles which they continue; this permits 

 many muscle attachments to a relatively 

 small surface. 



Nerve Supply. — The nerves of mus- 

 cles are derived from both sensory and 



motor nerve centers in the brain and spinal cord. The sensory 

 nerves convey sensations of muscle tension and condition to the 

 brain, but their endings in the muscle are not very sensitive to 

 mechanical injury. The motor nerves supply impulses originat- 



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Fig. 11. — Portions of 

 four fibers of skeletal mus- 

 cle. If carried out to their 

 ends these would be several 

 feet long for they are 

 often as much as 500 times 

 longer than they are in 

 diameter. Two are crossed 

 to suggest their relative 

 transparency. The one at 

 the right has been injured 

 and the sarcolemma re- 

 mains spanning a break in 

 the striated protoplasm. 

 (Stiles, HumanPhysiqlogy.) 



