218 THE MECHANICAL FUNCTIONS. 



sometimes in immediate contact, and forming 

 with it one and the same mass : at other times 

 placed at a distance, in a triangular cavity, 

 amongst the teeth of the hinge. The closing of 

 the valves produces, in all cases, a compression 

 of the cartilage, the elasticity of which tends, 

 therefore, to separate the valves from each other ; 

 that is, to open the shell. 



During the life of the animal, the usual and 

 natural state of its shell is that of being kept 

 open for a little distance, so as to allow of the 

 ingress and egress of the water necessary for its 

 nourishment and respiration. But as a security 

 against danger, it was necessary to furnish the 

 animal with the means of rapidly closing the 

 shell, and retaining the valves in a closed state. 

 These actions being only occasional, yet requir- 

 ing considerable force, are effected by a mus- 

 cular power : for which purpose sometimes one, 

 sometimes two, or even a greater number, of 

 strong muscles are placed between the valves, 

 their fibres passing directly across from the inner 

 surface of the one to that of the other, and firmly 



attached to both. 

 They are named, from 

 their office of bring- 

 ing the valves to- 

 wards each other, 

 the adductor muscles. 

 Fig. 102, which re- 

 presents the section of an oyster, shows the situa- 



