84 THE HUMAN BODY 



Cardiac Muscular Tissue. This consists of nucleated branched 

 cells which unite to form a network, in the interstices of which 

 blood-capillaries and nerve-fibers run. The 

 cells present transverse striations, but not so 

 distinct as those of the skeletal muscles, and 

 are said to have no sarcolemma (Fig. 49). 



The Chemistry of Muscular Tissue. The 

 chemical structure of the living muscular fiber 

 is unknown, but some idea as to it may be ob- 

 tained from examination of the substances it 

 yields on proximate analysis. Muscle contains 

 75 per cent of water; and, among other inor- 



FIG. 49. Cardiac \ . 



muscular tissue, mag- game constituents, phosphates and chlorides 



nified about 400 diam- . ,. , . , T7 , 



eters. The cell-bound- f potassium, sodium, and magnesium. When 

 n a the at rest a livin S muscle is feebly alkaline, but 



right-hand portion of after hard work, or when dying, this reaction 

 is reversed through the formation of sarco- 

 lactic acid (C 3 H 6 O 3 ) . Muscles contain small quantities of grape- 

 sugar and glycogen, and of nitrogenous extractives, especially 

 creatine (C 4 H 9 N S O 2 ). As in the case of all other physiologically 

 active tissues, however, the main post-mortem constituents of the 

 muscular fibers are protein substances. 



At least three proteins have been obtained from mammalian 

 striped muscle, myogen, a globulin, myosin, an albumin, both 

 coagulable by heat, and a protein which is insoluble in pure water 

 or dilute saline solution and which appears to form a protein 

 framework within the fiber. This latter is called the muscle 

 stroma and constitutes nine per cent of the weight of striated 

 muscle. Muscle tissue contains three or four times as much 

 myogen as myosin. Both of these proteins possess the property 

 of passing over into insoluble forms known respectively as myogen 

 fibrin and myosin fibrin. It has been commonly supposed that 

 the death stiffening, rigor mortis, which is such a marked feature 

 of the death of muscle tissue is due to this change of the muscle 

 proteins from a soluble to an insoluble form. 



Heart muscle contains relatively much less myogen and myosin 

 and much more stroma than does ordinary striated muscle, its 

 stroma constituting 56 per cent of its weight. Smooth muscle con- 

 tains an even larger proportion of stroma, 72 per cent. 



