The arterial layer includes the external carotid artery, which enters the parotid space after passing deep to the posterior belly of the digastric muscle (Fig. 3). At this point the external carotid artery gives rise to the posterior auricular artery, which gives off a stylohyoid branch that enters the stylomastoid foramen. This blood vessel is usually superficial to the facial nerve trunk as it exits from the skull by way of this same foramen. The posterior auricular artery then
continues posteriorly, running under cover of, and parallel to, the superior edge of the posterior belly of the digastric muscle. One should note that this muscle passes superficial to and protects almost all of the structures passing between the submandibular triangle superiorly and the carotid triangle inferiorly. This includes the internal jugular vein and the internal carotid artery in the carotid sheath, the last four cranial nerves, and the external carotid artery. The retromandibular vein or its branches, the cervical branch of the facial nerve, and the greater auricular nerve, however, pass superficial to the posterior belly of the digastric muscle.

The external carotid artery then pierces the medial surface of the parotid gland, and when it reaches the neck of the condylar process of the mandible it ends by giving rise to the maxillary artery and the superficial temporal artery. The maxillary artery passes medial to the condylar process of the mandible and enters the infratemporal fossa. The superficial temporal artery continues superiorly, accompanied by superficial temporal veins. The transverse facial artery, which arises from the proximal part of the superficial temporal artery, courses just superior to the parotid duct. The superficial temporal artery then enters the temporal region after passing between the external acoustic meatus and the temporomandibular joint. Accompanying these vascular structures at this level is the auriculotemporal nerve, which arises from the mandibular branch of the trigeminal nerve in the roof of the infratemporal fossa. It provides sensory innervation to the external acoustic meatus, external surface of the tympanic membrane, and temporomandibular joint, and cutaneous sensory innervation to the auricle and temporal scalp region.

The venous plane is superficial to the arterial plane (Fig. 3). It includes the retromandibular vein and its branches. The retromandibular vein is created by the union of the superficial temporal and maxillary veins. The maxillary vein is formed by the union of veins that are part of a large plexus of veins surrounding the lateral pterygoid muscle. This venous plexus communicates with veins of the face as well as with the cavernous sinus within the skull. The parotid veins also communicate with the pterygoid venous plexus. This pterygoid venous plexus in turn represents a potential pathway for the spread of superficial cutaneous infections to the cavernous sinus. This is a potentially lethal condition.

The retromandibular vein passes inferiorly through the substance of the parotid gland and is found between the branches of the facial nerve and the arterial layer. The retromandibular vein terminates at the lower edge of the gland by giving off anterior and posterior branches. The posterior branch joins with the posterior auricular vein to form the external jugular vein, which continues inferiorly, superficial to the posterior belly of the digastric muscle, then passes obliquely across the sternocleidomastoid muscle. It is frequently found just anterior to the great auricular nerve, which is on its way to the skin overlying the parotid gland. The anterior branch of the retromandibular vein unites with the anterior facial vein, forming the common facial vein. This passes inferiorly, superficial to the digastric muscle, to empty into the internal jugular vein.

The nerve plane is created by the facial nerve and its branches (Fig. 4). The facial nerve (seventh cranial nerve) exits from the skull through the stylomastoid foramen. At this point it is found with the stylomastoid branch of the posterior auricular artery, which enters the foramen and supplies the mucosa of the tympanic cavity, the mastoid cells, and the semicircular canals. The artery is usually superficial to the facial nerve. After emerging from the foramen, two branches arise from the facial nerve before it enters the parotid gland. The first branch, the posterior auricular nerve, provides innervation to the posterior auricular muscles and the intrinsic muscles of the auricle. The second branch, arising from the extraparotid portion of the facial nerve, provides motor innervation to the posterior belly of the digastric and stylohyoid muscles. This portion of the nerve, approximately 1 cm in length, then penetrates the posteromedial surface of the parotid gland. This nerve trunk passes forward in the glandular parenchyma for a distance of approximately 1 cm or less, then divides into two branches—a larger temporofacial branch, which creates temporal and zygomatic nerves, and a smaller cervicofacial branch, which gives rise to buccal, marginal mandibular, and cervical nerves. An isthmus of glandular tissue separates the temporofacial branch from the cervicofacial branch.

The facial nerve provides motor innervation to the muscles of facial expression. The platysma is included in this category of muscle. The nerve branches, as they pass through the parotid gland, divide it into two portions. The part of the gland that is superficial to the nerves is referred to as the superficial (exofacial) lobe, and the portion of the gland that is internal to the nerve layer is referred to as the deep (endofacial) lobe. Multiple communications are found between the nerve branches as they pass through the gland. The zygomatic and temporal nerves are frequently multiple, whereas the mandibular and cervical branches are often single. The cervical and mandibular branches can extend below the mandible, whereas the cervical branch passes superficial to the posterior belly of the digastric muscle. The nerves become more superficial as they pass distally.