196 THE MKCHANICAL FUNCTIONS. 



properties of cartilage, being highly elastic, and 

 formed of parallel series of condensed transverse 

 fibres, directed from the hinge of one valve to the 

 similar part of the other, and having generally a 

 deep black colour, and a pearly bistre. The carti- 

 lage is always situated within the ligament ; some- 

 times 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 nourish- 

 ment and respiration. But as a security against 

 danger, it was necessary to furnish the animal with 

 the means of rapidly closing the shell, and retain- 

 ing the valves in a closed state. These actions 

 being only occasional, yet requiring considerable 

 force, are effected by a muscular power ; for which 

 purpose sometimes one, sometimes two, or even a 

 greater number of strong muscles are placed be- 

 tween the valves, their fibres passing directly across 

 from the inner surface of the one to that of the 

 102 other, and firmly at- 



tached to both. — They 

 are named from their 

 ofiice of bringing the 

 valves towards each 

 other, the adductor mus- 

 cles. Fig. 102, wliich re- 



