100 THE HUMAN BODY 



the energy exhibited by a contracting muscle is derived from 

 the oxidation of fuel substances within it. The physical accom- 

 paniments of oxidation are not the same in the two cases; the 

 fuel under the boiler burns with flame and at a high temperature; 

 the fuel substance within the muscle burns without flame and at 

 a temperature only slightly higher than that of the body. The 

 energy yield, however, for corresponding amounts of fuel is as 

 great in one case as in the other. The fuel substance used by 

 contracting muscle is probably for the most part a certain sugar, 

 dextrose, or its anhydride, glycogen. When dextrose is completely 

 oxidized it yields carbon dioxid and water. This reaction is rep- 

 resented by the equation C 6 H 12 O 6 +12O=6CO 2 +6H 2 O. 



The manner in which this energy of oxidation is converted 

 within the muscle to energy of motion is not certainly known, 

 although many interesting theories have been proposed to 

 explain it. Most physiologists agree that the mechanism of 

 skeletal muscle is quite different from that present in smooth 

 muscle. 



The longitudinal fibrils which form such a characteristic feature 

 of skeletal muscle are believed by many physiologists to repre- 

 sent the actual contractile elements of this type of muscular 

 tissue. The precise way in which they perform their function is 

 at present a matter of conjecture. It has been suggested that 

 contraction is due to a rush of fluid into the fibrils from the sur- 

 rounding sarcoplasm. Those who hold this view believe that the 

 oxidation of dextrose to carbon dioxid and water bring about 

 conditions within the muscle which result in movement of fluid 

 of the sort indicated. 



Physiology of Smooth Muscular Tissue. What has hitherto 

 been said applies especially to the skeletal muscles; but in the 

 main it is true of the unstriped muscles. These also are irritable 

 and contractile, but their changes of form are much slower than 

 those of the striated fibers. Upon stimulation, a longer period 

 elapses before the contraction commences and when, finally, this 

 takes place it is comparatively very slow, gradually attaining a 

 maximum and gradually passing away. 



Unstriped muscular tissue has a remarkable power of remain- 

 ing in the contracted state for long periods: the muscular coats 

 of many small arteries, for example, are rarely relaxed; some- 



