ELASTIC PROPERTIES 



It has long: been known tluit the hganient per- 

 forms a mechanical function by automatically 

 pushino- the vah-es apart when the tension of the 

 adductor nniscle relaxes. In a live oyster, how- 

 ever, tlie gaping of the valves never attains the 

 potential maximum limited by the angle and length 

 of the beaks. This can t)e demonstrated by a 

 simple test: if the entire adductor muscle is 

 severed, the valves open to a much greater angle 

 than that maintained l\v a full}' narcotized oyster 

 with a completely relaxed muscle attaclied to the 

 shell. It follows from this observation thtit during 

 the entire life of the oyster the adductor muscle, 

 even at the periods of its greatest relaxation, exerts 

 a certain pulling force against the elastic tension 

 of the ligament. 



In view of the voluminous literature dealing 

 with tlie structure and function of bivalve muscles 

 it is surprising to find how little attention has been 

 given to the study of the physical properties of 

 the ligament. The first attempt to determine the 

 pulling force of the muscle sufficient to counteract 

 the elasticity of the ligament was made in a rather 

 crude manner in 1865 by Vaillant who tried to 

 measure the elastic force of the ligament of 

 Tridacna shells. Trueman (1949) erroneously 

 gives credit for this pioneer work to Marceau 

 (1909), who only repeated the method used by 

 earlier investigators (Plateau, 1884). 



After removing the soft body of Tridacna, 

 Vaillant set the empty shell on a table with the 

 flat valve uppermost and placed a glass graduate 

 on top of it. Water was poured into the graduate 

 until the valves closed. Then the volume of 

 water was read and its weight computed. The 

 weight of the water plus the weight of the glass con- 

 tainer and of the valve gave Vaillant a value which 

 he called the resistance of the ligament. For a 

 shell of Tridacna, apparently one of small size, he 

 gives the following figures: weight of water re- 

 quired to close the valves — 250 g. ; weight of the 

 vessel — 700 g. ; weight of the valve — 632 g. The 

 total force needed to overcome "the resistance" of 

 the ligament is, therefore, 1,582 g. 



A similar method was used by -Plateau (1884), 

 the only differences being that weights were added 

 to a metal pan suspended from a lof)p encircling 

 the valves, as sliown in figure 65, and tliat the 

 shell was placed on a metal ring. The elastic 

 force exerted by the ligaments of several common 

 bivalves, as determined by Plateau, was found to 



FiouRE 6.5. — Plateau's method of measuring elasticity of 

 the ligament. 



be as follows: Oxtrea edulis — 333.8 g.; Venus 

 verrucosa — 500.0 g.; Mya arenan'a — 620.0 g.; and 

 Mytdus edulis — 1,051.8 g. In Marceau's paper 

 of 1909 the data taken from Plateau's work are 

 repeated without change or verification. 



Trueman's investigation of the ligament of 

 Tellina (1942) marks a renewal of interest in the 

 study of the physical properties of the ligament. 

 In a later paper (1951) he finds that in very j'oung 

 0. edulis the outer layer of the ligament (ten- 

 silium according to our terminology) forms a 

 continuous band along the entire dorsal margin 

 of the hinge, but that in adults this outer layer 

 separates into the anterior and posterior portions, 

 leaving the inner layer (resilium) exposed at the 

 dorsal edge. The axis about which the valves of 

 the adult 0. edulis open (pivotal axis) is the same 

 in C. rirginica (figiu-e 54, piv. ax.). In the closed 

 shell of Osfrea and Crassosfrea the central part of 

 the ligament (the resilium) is under compression 

 and the two flanking portions (tensilium or outer 

 layer of Trueman) are under tension. 



To measure the opening moment of thrust of a 

 iiinge ligament, Trueman (1951), uses the fol- 

 lowing method, sliown diagrammatically in figiu'e 

 66: soft parts of tlie body are removed and the 

 lower valve embedded in plasticine; a counter- 

 balanced beam is erected above the valve in such 

 a way that the weight placed on the pan at the 

 left end is applied at the center of the upper 

 valve. The distance from the left end of the 

 l)eam to the arm touching the centroid of the 



THE LIGAMENT 



59 



