302 ANIMAL BIOCHEMISTRY 



their chemical properties, they have many chemical characteristics in 

 common. The three types of muscles are (1) voluntary (striated, 

 skeletal), (2) involuntary (smooth, unstriated), and (3) cardiac. 



Voluntary muscle acts imder conscious control. It is cross-striated 

 with horizontal bandings quite visible under the microscope. The 

 dark and light areas arise from differences in light refraction in 

 the banded areas due to structural differences. The striated muscle 

 constitutes the major fraction of muscle tissue. It contains fibers made 

 up of long, spindle-shaped, nucleated cells held together by connective 

 tissue fibers and enclosed in a connective tissue sheath to form bundles 

 called myofibrils. Involuntary muscles are smooth and non-striated. 

 They are composed of short, spindle-shaped, nucleated cells. These 

 cells are also held together in a compact group by connective tissue 

 fibers. Smooth muscles make up the skin and the walls of the bladder, 

 arteries, and veins. Cardiac muscle is made up of irregular, sometimes 

 branched cells which are striated in a manner similar to the voluntary 

 muscles. The cells are truncate in appearance with a protoplasmic 

 connection between cells. 



Blood vessels are distributed throughout the muscle tissue in a fine 

 web of capillaries so that each muscle fiber comes into close contact 

 with them. Lymph surrounds and bathes every muscle cell, aiding in 

 the transfer of nutrients to and waste products from the muscle fiber. 

 Nerve endings are also found in each cell, thus linking the control 

 center with the site of action. 



Muscle Proteins 



As is true of most biological substances, muscle is composed largely 

 of water, about 70 to 75 per cent. Protein accounts for about 20 to 25 

 per cent of the fresh weight, with lipides, glycogen, glucose, and 

 products of intermediate metabolism accounting for the remaining 

 weight (about 5 per cent). Muscle proteins can be separated into five 

 general types, myogen, myosin, globulin X, stroma, and actin. Myosin 

 is soluble in salt solutions at pH 6.3 and an ionic strength of 0.62. 

 Actin can be removed by salt solutions of similar pH but lower ionic 

 strengths. Globulin X is extracted with salt solutions and precipitates 

 when the salts are removed by dialysis. No function is evident for 

 this protein. Stroma, the protein that remains after extraction of the 

 fractions mentioned, forms the membranes of the muscle cells. The 

 red color of muscles is due to small amounts of myoglobin, an iron- 

 porphyrin-histone complex with properties similar to those of hemo- 

 globin (page 293). 



Myosin and actin form the contractile portion of muscle. Actin, 



