Lab 19 Pre-Lab Exercise:
1. Bony landmarks of the face
Skeletal landmarks in the head and neck are used for locating major blood vessels, glands, muscles, and for locating points of access to the airway. Neurological examination of the cranial and upper cervical nerves is carried out by assessing function in the head and neck. In addition, information about the general status of body health can often be obtained by evaluating surface features, the eye, the oral cavity, and the characteristics of speech.
The head is in the anatomical position when the inferior margins of the bony orbits and the superior margins of the external acoustic meatuses are in the same horizontal plane (Frankfort plane).
Head of the mandible: anterior to the external ear and behind and inferior to the posterior end of the zygomatic arch. It is best found by opening and closing the jaw and palpating the head of the mandible as it moves forward onto the articular tubercle and then back into the mandibular fossa, respectively.
Zygomatic bone and arch: the arch extends forward from the region of the temporomandibular joint (TMJ) to the zygomatic bone, which forms a bony prominence lateral to the inferior margin of the anterior opening of the orbit. The parotid duct extends across the masseter muscle just inferior to the zygomatic arch. The duct turns medially to pierce the buccinators and opens into the oral vestibule.
Mastoid process: is a large bony protuberance that is easily palpable posterior to the inferior aspect of the external acoustic meatus. The superior end of the sternocleidomastoid muscle attaches to the mastoid process.
External occipital protuberance: is palpable on the midline posteriorly where the contour of the skull curves sharply forward. This landmark marks the point superficially where the back of the neck joins the head.
Vertex: is the highest point of the head in the anatomical position and marks the approximate point on the scalp where there is a transition from cervical to cranial innervation of the scalp. Anterior to the vertex the scalp and face are innervated by the trigeminal nerve (CN V). Posterior to the vertex, the scalp is innervated by branches from cervical spinal nerves.
2. Major features and soft tissues of the face
The major features of the face are those related to the anterior openings of the orbit, the nasal cavities, and the oral cavity.
The palpebral fissures are between the upper and lower eyelids and can be opened and closed. The oral fissure is the gap between the upper and lower lips and can also be opened and closed. The sphincter muscles of the oral and palpebral fissures are the orbicularis oris and orbicularis oculi muscles, respectively. The muscles are innervated by the facial nerve (CN VII).
The nares are the anterior apertures of the nasal cavities and are continuously open. The vertical groove in the midline between the external nose and the upper lip is the philtrum. Sensory innervation of the face is carried by the trigeminal nerve (CN V). The three divisions of this nerve are represented on the face and can be tested by touching the forehead (ophthalmic nerve CN V1), the anterior cheek (maxillary nerve CN V2), and the skin over the anterior body of the mandible (mandibular nerve CN V3).
Figure below shows actions of selected facial muscles of facial expression.
The facial nerve (CN VII) innervates the muscles of facial expression. After emerging from the stylomastoid foramen, the main stem of the facial nerve has posterior auricular, digastric, and stylohyoid branches, the parotid plexus gives rise to temporal (T), zygomatic (Z), buccal (B), marginal mandibular (M), cervical (C), and posterior auricular branches. These branches form a plexus within the parotid gland, the branches of which radiate over the face, anastomosing with each other and the branches of the trigeminal nerve (CN V).
3. Estimating the position of the middle meningeal artery
The middle meningeal artery is a branch of the maxillary artery in the infratemporal fossa. It enters the skull through the foramen spinosum and is within the dura mater lining the cranial cavity. The anterior branch of the middle meningeal artery is part of the vessel most often torn. This branch is in the temple region of the head, approximately midway between the superior margin of the orbit and the upper part of the external ear in the pterion region. The pterion is a small circular area enclosing the region where the sphenoid, frontal, parietal, and temporal bones of the skull come together. It can be located by two fingers breadth superior to the zygomatic arch and one thumb breadth posterior to the frontal process of the zygomatic bone (approximately 4 cm superior to the midpoint of the zygomatic arch).
Lateral blows to the head can fracture the internal table of bone of the skull and tear the middle meningeal artery in the outer layer of dura mater that is fused to the cranium. Blood under pulsatile arterial pressure leaks out of the vessel and gradually separates the dura from the bone, forming progressively larger extradural (epidural) hematoma. Untreated middle meningeal hemorrhage may cause death in a few hours.