Final answer:
The harpoon is the part of a standard syringe that penetrates the rubber stopper of the dental cartridge, which helps to inject the anesthetic fluid.
Step-by-step explanation:
The part of the standard syringe that penetrates the rubber stopper of the dental cartridge is the harpoon. The harpoon is used to firmly engage the rubber stopper or plunger of the anesthetic cartridge. This engagement allows the dental professional to inject the anesthetic fluid into the patient's tissue through the needle that is attached to the other side of the cartridge. The needle adapter is the part where the needle is attached to the syringe, but it does not penetrate the stopper. The finger grip and the thumb ring aid the dentist in holding and controlling the syringe during injection but are not involved in penetrating the stopper.