8 : 2/ Muscles 



139 



function, of innervation, of body location, of embryological develop- 

 ment, and of histology. The histologic classification is the most widely 

 used and probably the least ambiguous. Histologically, one can dis- 

 tinguish, in the vertebrates, two types of muscles : striated and smooth. 

 Striated muscle, when viewed under the microscope, appears to have 

 alternate dark and light bands distributed in a regular pattern across 

 long fibers. Smooth muscle consists of shorter fibers with no striations. 

 Striated muscles form a large portion of our meat diet. If one 

 examines a piece of steak, one notes there are large bundles or sub- 

 divisions of the muscle. The entire muscle is surrounded by a sheath 

 of connective tissue. Between the large bundles comprising the muscle 

 run connective tissues, blood vessels, and nerves. Each large bundle is 

 then divided into smaller bundles, and each of these is finally subdivided 



H 



I 



Mitochondrion 



I 



* 



m ■ 



I 



I 





VY 



Sarco/emma 



m 



I0|JL. 



Nucleus 



Figure I. Diagram of striated muscle fiber. Each fiber con- 

 tains many nuclei and mitochondria. In general, the fiber is 

 not as straight as shown in the diagram. The different bands 

 are characterized as follows. The A Band stains dark and is 

 anisotropic (birefringent) ; it is also called the Q disc. The / 

 Band stains less and is isotropic; it is also called the J disc. 

 The Z disc, in middle of/ band, stains darkly. The H zone is 

 the less stained region in middle of A band. 



into "muscle fibers." The major portion of the striated muscle is made 

 up of these fibers, 10-100 /x in diameter, and of lengths that reach 100 cm 

 or more in the larger vertebrates. A piece of the fiber under high 

 magnification would look something like Figure 1. Each fiber is 

 crossed by a number of bands, each with its own name. 



The ends of the fibers of many striated muscles are attached to 

 tendons. Throughout the length of the muscle fiber run still smaller 

 fibers called myofibrils. These possess the same characteristic striations 

 of the original muscle fibers. For reasons not at all understood, the 

 corresponding bands of adjacent myofibrils are lined up with one 

 another, thereby causing the striation of the entire muscle fiber. Besides 

 the fibrils, a striated muscle fiber contains several other organelles and 



