STRUCTURE OF MUSCLE 35 



globin), extractives, creatine, hypoxanthine and xanthine, 

 fats, glycogen, inosite (the so-called muscle-sugar, but in 

 reahty a benzene derivative), and lactic acid. 



When muscle loses its blood-supply it soon undergoes 

 a profound physical change. From being translucent and 

 elastic it becomes opaque and stil!. This alteration, hke 

 the clotting of muscle plasma, is accompanied by a develop- 

 ment of sarcolactic acid. The condition which the muscle 

 assumes is termed rigor mortis. A similar change may be 

 brought about if the muscle is slowly warmed above the 

 coagulation temperature of its proteins. Since the most 

 striking chemical change is the development of lactic acid, 

 the question arises whether the presence of this acid is the 

 cause or the result of the physical alteration in the muscle. 

 Lactic acid increases in muscle as the result of activity, 

 and the rate of onset of rigor is dependent upon the degree 

 of accumulation of the acid. Further, rigor can be pre- 

 vented even in a dead muscle if the accumulation of acid 

 is prevented by perfusion. The formation of lactic acid, 

 then, would seem to be the forerunner and the cause of 

 rigor. 



Structure of Muscle 



Unstriated muscle is composed of fusiform cells of 

 variable length. There is an oval nucleus. The sarco- 

 plasm is occupied with fibrils disposed longitudinally. 



Striated muscle consists of fibres of 0-05 mm. diameter and 

 of varying length up to 3 cm. Each fibre is enveloped in an 

 elastic sheath, the sarcolemma. It is composed of discs 

 or sarcomeres of dark and Ught material alternately— an 

 arrangement which gives to this type of muscle its name. 

 In the middle of each Ught band is a row of granules con- 

 stituting the so-called Krauses membrane. The complete 

 disc therefore consists of a dark middle portion and a light 

 portion at each end, Krause's membrane being the surface 

 of union of adjacent discs. Each of these discs is broken 

 up longitudinally into a number of longitudinal fibrillse, 



