116 THE HUMAN BODY 



gastrocnemius, arouses a strip of stomach muscle to pronounced 

 activity. 



Mechanism of Contraction of Smooth Muscle. The structure 

 of smooth muscle is, as shown formerly, much less complex than of 

 skeletal muscle. No elaborate system of sarcostyles, sarcoplasm, 

 and sarcolemma exists. The spindle-shaped cells with their en- 

 vironment of lymph are the contractile elements. In fact when 

 we attempt to compare smooth muscle with skeletal we find that 

 the smooth muscle-cell corresponds better to the sarcostyle than to 

 the fiber, although the fiber is the cell unit. The lymph which 

 bathes the cell of smooth muscle functions toward it as does the 

 sarcoplasm toward the sarcostyle. Contraction of smooth muscle 

 depends, then, on interaction of muscle-cell with lymph, as in skele- 

 tal muscle on interaction of sarcostyle with sarcoplasm. This latter 

 interaction is of such a sort that to keep the sarcostyles in a state 

 of contraction a continuous expenditure of energy is necessary, 

 and fatigue is bound ultimately to occur. The expenditure of 

 energy in smooth muscle, on the other hand, appears to take place 

 only during an actual change in length, whether the change is a 

 shortening or a lengthening; the maintenance of a given length 

 after it is once attained seems not to require further energy libera- 

 tion. This difference accounts for the ability of smooth muscle to 

 continue in contraction without fatigue. 



The suggestion made above, that energy expenditure occurs 

 during change in length in smooth muscle, even though the change 

 be a relaxation, is in harmony with the interesting fact that most 

 smooth muscle tissues are supplied with two sets of nerves. Stim- 

 ulation by way of one set induces contraction; by way of the other, 

 relaxation. This is in marked contrast with the situation in 

 skeletal muscle, where the only function of stimulation is to arouse 

 contraction; relaxation following spontaneously upon the release 

 from excitation. 



Heat rigor in smooth muscle shows an interesting difference 

 from the same phenomenon in skeletal muscle. In the latter 

 tissue the result of heating above the death point is a pronounced 

 contraction. When smooth muscle, is thus heated, instead of 

 contracting it undergoes marked relaxation. We know that 

 when skeletal muscle is heated there is production of lactic acid 

 within it, and that this lactic acid brings about the shortening 



