Which structure passes through the substance of the parotid gland and divides it into superficial and deep lobes?

The facial nerve (cranial nerve VII) passes through the parotid gland as it exits the stylomastoid foramen and divides into its five terminal branches (temporal, zygomatic, buccal, marginal mandibular, and cervical) within the parotid gland. This anatomical relationship is critically important surgically: during parotidectomy, preservation of the facial nerve is paramount to avoid facial paralysis. The original answer stated 'Maxillary artery' which is incorrect — the maxillary artery passes through the infratemporal fossa, not the parotid gland. The retromandibular vein and external carotid artery also pass through the parotid, but the facial nerve is the defining clinical relationship.