Responses of Higher Animals: The Effectors - 437 



compares very favorably with the 10 percent 

 efficiency of the best steam engine. This 

 means that under optimal conditions, 60 per- 

 cent of the energy expended by a muscle 

 appears as heat, and 40 percent as work. 

 However, the heat put forth by the muscles 

 tluring work is not wasted, since this heat 

 contributes in large measure to the mainte- 

 nance of the body temperature. The body 

 does not possess any specialized effectors to 

 function as heat producers, but the muscles 

 are thrown into a special sort of action, 

 namely shivering, whenever the body loses 

 more heat than it gains from the general 

 metabolism. None of the metabolic reactions 

 throughout the body is 100 percent efficient 

 in the fulfillment of its special end, and con- 

 sequently heat is a by-product given off by 

 all the cells; and since muscle constitutes 

 the bulk of the tissues, the heat produced by 

 the muscles plays a dominant role in main- 

 taining the temperature of the body. 



Fine Structure of the Contractile Mech- 

 anism. The banded pattern of single fibers 

 in striated (skeletal and cardiac) muscles has 

 been observed for many years, but a precise 

 knowledge of the nature and significance of 

 this pattern remained obscure until about 

 1957. Then two English workers, H. E. Hux- 

 ley and A. F. Huxley, 1 simultaneously and 

 independently, began to report their studies 

 on the fine structure of muscle; their studies 

 had utilized various modern techniques, such 

 as electron microscopy and x-ray diffraction 

 analysis, as well as interference and phase 

 microscopy. 



Previously it was thought that myofibrils 

 were continuous strands of protein material 

 that extended long distances along the 

 length of the muscle fiber, and that a striated 

 pattern resulted from alternating segments 

 of denser material, regularly spaced and 

 evenly aligned across the fiber (Fig. 24-6). 



1 These two Huxleys are not relatives. 



Fig. 24-7. Fine structure of a skeletal muscle fiber (rabbit): electronmicrograph, magnified 24,000 diameters. 

 Compare this photograph with the diagram of Figure 24-8. Be sure to identify the thick and thin filaments 



Compare this phorograpn witn ine aiagram or rigure Z4-o. Be sure to laennry ine inicK ana rnin Tiiamenrs 

 and the H-, A-, and Z-bands (see text description, p. 438). (Courtesy of H. E. Huxley and the University 

 Postgraduate Medical School, Cambridge, England.) 



