160 



THE MECHANICAL FUNCTIONS. 



a deep black colour, and a pearly lustre. The cartilage is 

 always situated within the ligament, sometimes in immediate 

 contact, and forming with it one of 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. 



Durins: 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 insfress and esiress of the water neces- 

 sary 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 occa- 

 sional, yet requiring considerable force, are effected by 

 a muscular power: for which purpose sometimes one, some- 

 times 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 bringing the 

 valves towards each other, 

 the adductor muscles. Fig. 

 102, which represents the sec- 

 tion of an oyster, shows the 

 situation of the hinge l, the 

 adductor muscles a, and the transverse direction of its fibres, 

 with respect to the valves. When these muscles are not in 

 action, the elasticity of the cartilage attached to the hinge is 

 sufficient to separate the valves; but as they were not intend- 

 ed to open beyond a certain extent, it was necessary to pro- 

 vide some limitation to the action of the cartilage. The ad- 

 ductor muscle might, it is evident, be called into play to 

 counteract that action; but this would require a constant 

 muscular exertion, and a great expenditure, therefore, of 

 vital force. Nature has always shown a solicitude to econo- 



