TONUS OF MUSCLE 449 



Muscular system. — The shell is closed and kept closed 

 by the action of the two adductor muscles. When these 

 are relaxed under nervous control, the elasticity of the hinge 

 ligament opens the valves. The foot is a muscular protru- 

 sion of the ventral surface, under the control of three 

 muscles — a retractor and a protractor anteriorly, and a 

 posterior retractor. Its upper portion contains some coils 

 of gut and the reproductive organs ; its lower region is very 

 muscular. The protrusion or extension of this locomotor 

 organ is mainly due to an inflow of blood, which is pre- 

 vented from returning by the contraction of a sphincter 

 muscle round the veins. In moving, the animal literally 

 ploughs its way along the bottom of the pond or river pool, 

 and leaves a furrow in its track. The muscle fibres, as in 

 the snail, are mainly of the slowly contracting non-striped 

 sort, but those of the adductor and of the heart show 

 oblique cross-striping. In that part of the adductor muscle 

 of Pecten (and some other bivalves) that effects the rapid 

 closing of the valves, and hence the swimming, the muscle- 

 fibres are transversely cross-striped, and the same is true of 

 those found in the margin of the mobile mantle. There is 

 here therefore a good instance of the connection between 

 striation and rapidity of contraction and relaxation. 



Tonus. — The adductor muscles of bivalves furnish a 

 good example of a remarkable property of smooth muscle, 

 that of maintaining a state of contraction with little or no 

 expenditure of energy. If a piece of wood is inserted 

 between the open valves, the adductors contract, and the 

 valves close as far as they can. If the wood is twisted out, 

 the valves remain as they were, without closing : it is 

 impossible to pull the valves apart without tearing the 

 muscle, but they can easily be pressed closer together, 

 when they maintain their new position as firmly. The 

 mechanism recalls that of a catch or ratchet. Similar 

 behaviour is shown by smooth muscle in other Inverte- 

 brates (sea-anemones, for instance, maintain either the 

 contracted or the expanded state without expenditure of 

 energy and without fatigue), and in the hollow viscera of 

 Vertebrates. Striated muscle may behave in an analogous 

 way, but only under the constant controlling influence of the 

 nervous system. 



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