Final answer:
The maxillary vein is near the injection site behind the last maxillary molar, and the mandibular foramen is where the dentist must inject anesthesia to numb the lower teeth, which is anterior to it and near the lingula.
Step-by-step explanation:
The venous plexus that is near the injection site behind the last maxillary molar is related to the maxillary vein, which drains blood from the maxillary region and is part of the network of veins that eventually leads to the external jugular vein. When performing dental anesthesia for procedures on the lower teeth, a dentist must inject the anesthesia near the mandibular foramen, which is where the sensory nerve and blood vessels that supply the lower teeth enter and run down the mandibular body. This injection site is typically located on the lateral wall of the oral cavity, just anterior to the mandibular foramen and close to the lingula, a small bone flap that is medial to the ramus where a ligament that helps in jaw movement attaches.