The valves are distinguished as dorsal and ventral.
Valves to move forward h, Central muscles (close valves).
The two valves of the shell of the common cockle are similar to each other, and somewhat circular in outline.
To regulate the heat it is necessary either to instal a number of small radiators or to divide the radiators into sections, each section controlled by distinct valves; steam may then be admitted to all the sections of the radiator or to any less number of sections as desired.
The perforation of the paper when done by hand is usually performed by means of small mallets, but at the central telegraph office in London, and at other large offices, the keys are only used for opening air-valves, the actual punching being done by pneumatic pressure.