THE LIGAMENT. 291 



THE HINGE OF BIVALVE SHELLS. 



Under this term is included all the apparatus by which 

 the two valves are joined together, and act upon each other, 

 so as to keep their relative positions. It is placed on the 

 dorsal margin, and includes the lateral and cardinal teeth, 

 the cartilage and ligament, and the pit and the fulcrum, 

 and, in some cases, other processes and appendages-. 



THE LIGAMENT, 



Using the word comprehensively, includes two distinct sul> 

 stances, which are sometimes so united as to appear but one ; 

 and sometimes they are quite separated, and their functions 

 are very different. The ligament, properly so called, is the 

 outer horny substance, which serves to bind the valves 

 together by a portion of their dorsal margin. The other 

 portion, now commonly distinguished as the cartilage, is a 

 very elastic material, which, unless pressed, would occupy 

 more space than that allotted to it, and therefore tends to 

 press outwards so as to open the valves. It thus presents a 

 force opposed to the adductor muscles of the animal, which 

 tend to bring the valves together. In some cases the car- 

 tilage lies close within the ligament, on the outer edge of 



