The aspirating syringe has a device such as a sharp, hook-shaped end (often called the harpoon) attached to the piston that is used to penetrate the thick silicone rubber stopper (bung) at the opposite end of the cartridge (from the needle). Provided the needle is of adequate gauge, when negative pressure is exerted on the thumb ring by the administrator, blood will enter into the needle and will be visible in the cartridge if the needle tip rests within the lumen of a blood vessel. Positive pressure applied to the thumb ring forces local anesthetic into the needle lumen and the tissues wherever the needle tip lies. The thumb ring and finger grips give the administrator added control over the syringe. Almost all syringe manufacturers provide syringes with both "regular" and "small" thumb rings Most metallic, breech-loading, aspirating syringes are constructed of chrome-plated brass and stainless steel.
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