r-^jTSIf^; 



Microns 



Figure 147. — Two photomicrographs of a longitudinal section of the translucent part of the adductor muscle near the 

 valve (the left side of the photograph) . The muscle was preserved in a contracted state in Bouin with formalin solu- 

 tion. Note the thick, short contracted fibers on the left and the beginning of folding at the right edge of it. Con- 

 tracted fibers are deeply stained with eosin. The photomicrograph on the right shows folded fibers a short distance 

 away from the area of the same section shown at left. 



with the folded fibers and at theh' anchorage in 

 the connective tissue. Under this condition the 

 contracted portions will bring the valves together 

 and conapress the noncontracted fibers into folds. 

 This gives the oyster a considerable degree of 

 flexibility in controlling the degree of opening of 

 the valves. 



Observations by Bandmann and Reichel (1955) 

 on Pinna nobilis deal with similar conditions. In 

 the smooth muscle of this mollusk plastic length- 

 ening is combined with an orientation of the fiber 

 structure without any changes of its elastic prop- 

 erties. The reverse process (disorientation) takes 

 place during contraction, which is accompanied by 

 an increase in dynamic stiffness. The authors 

 attribute plastic and contractile length alterations 

 to two different mechanisms: change in orienta- 

 tion and change in molecular shape within the 

 contractile elements. 



No observations have l)een made in tlie living 

 oyster of the contractions of small bundles of 

 fibers that run parallel to the surface of the valves. 







0.3 



Millimeters 



Figure 148. — Longitudinal section of partially contracted 

 translucent portion of the adductor. Contracted fibers 

 appear as black spindles. Osmium fixation. Iron 

 hematoxylin. 



THE ADDUCTOR MUSCLE 



159 



