Head and neck: Overview and surface anatomy

Face and scalp

The superficial fascia of the scalp is firm, dense, fibroadipose, and closely adherent to the skin and to the underlying muscle, epicranius and the epicranial aponeurosis. Posteriorly, the fascia is continuous with the superficial fascia of the back of the neck, and laterally it is prolonged into the temporal region, where it is looser in texture. Three fascial layers (a subcutaneous fibroadipose tissue, a superficial musculo-aponeurotic system (SMAS) and the parotid–masseteric fascia) are recognized on the face superficial to the plane of the facial nerve and its branches. On the lateral side of the head, above the zygomatic arch, the temporoparietal fascia lies in the same plane as, but does not blend with, the superficial musculo-aponeurotic system. It is superficial to the temporal fascia and blends superiorly with the epicranial aponeurosis. The parotid gland is surrounded by a fibrous capsule derived from the deep cervical fascia.

Neck

The superficial cervical fascia is a zone of loose connective tissue between dermis and deep fascia and is joined to both. It contains a variable amount of adipose tissue and platysma but is hardly demonstrable as a separate layer. The deep cervical fascia is conventionally subdivided into three sheets (superficial investing, middle and deep layers) that surround the muscles and viscera of the neck to varying degrees, and the carotid sheath, a condensation of deep fascia around the common and internal carotid arteries, internal jugular vein, vagus and ansa cervicalis. The fascial layers of the neck define a number of potential tissue ‘spaces’ above and below the hyoid bone ( Ch. 37 ). In health, the tissues within these spaces are either closely applied to each other or are filled with relatively loose connective tissue. However, they offer potential routes by which infection may spread within the head and neck, and between the tissue spaces of the face and the mediastinum. They also offer convenient planes for almost bloodless dissection during surgery.

Skull and mandible

The skull is composed of 28 separate bones, most of them paired ( Ch. 34 ). It can be divided into the cranium, consisting of the calvaria (brain box) and basicranium, which together surround and protect the brain; a delicate facial skeleton composed mainly of thin-walled bones, some of which contain air-filled cavities that are known collectively as the paranasal sinuses; and the mandible. The cranial cavity contains the brain and the intracranial portions of the cranial nerves; the blood vessels that supply and drain the brain and the haemopoietic marrow of the overlying bones; the meninges (dura, arachnoid and pia mater); and the cerebrospinal fluid in the subarachnoid space. The cavity is incompletely divided by dural partitions, notably the falx cerebri, lying between the cerebral hemispheres, and the tentorium cerebelli, lying between the cerebellum and occipital lobes. Almost all of the venous blood from the brain and cranial bones drains via sinuses lying between the endosteal and meningeal layers of the dura mater into the internal jugular vein. Internally, the cranial base is divided into anterior, middle and posterior cranial fossae, which contain the frontal and temporal lobes of the cerebral hemispheres and the cerebellum, respectively. Foramina in the bones of the skull base and facial skeleton transmit neurovascular bundles that may be compromised at these sites by pathology or trauma.

The bony orbits contain the eyeballs, oculogyric muscles and lacrimal glands with their associated neurovascular supplies ( Chapter 44, Chapter 45 ). The temporal bones contain the inner, middle and external ears ( Chapter 42, Chapter 43 ). The maxillae are the largest of the pneumatized bones of the midface; they contain the maxillary air sinuses and bear the upper teeth. The mandible bears the lower teeth and articulates with the temporal bones at the temporomandibular joints. The skull articulates with the first cervical vertebra (atlas); movements of the skull on the cervical vertebrae occur at the atlanto-occipital joints.

The skull provides attachments for many muscles, including all the craniofacial muscles, the oculogyric muscles, the muscles that act on the temporomandibular joint, the superior constrictor of the pharynx, the muscles of the soft palate, all but one of the extrinsic muscles of the tongue, the muscles of the suboccipital region, and the cranial attachments of trapezius and sternocleidomastoid.

Cervical vertebrae

There are seven cervical vertebrae. They are the smallest of the movable vertebrae and are characterized by a disproportionately large vertebral canal. All but the seventh are also characterized by a foramen in each transverse process, the foramen transversarium (these foramina may be absent or sometimes duplicated in the seventh cervical vertebra). The first (atlas), second (axis) and seventh (vertebra prominens) cervical vertebrae are atypical.

Hyoid bone and laryngeal cartilages

The hyoid bone lies in the midline at the front of the neck at the level of the fourth cervical vertebra ( ). It is suspended from the styloid processes by the stylohyoid ligaments and gives attachment to the suprahyoid and infrahyoid groups of muscles. The skeletal framework of the larynx is formed by a series of cartilages interconnected by ligaments and fibrous membranes and moved by a number of muscles ( Ch. 41 ). The laryngeal cartilages are the single cricoid, thyroid and epiglottic cartilages, and the paired arytenoid, cuneiform, corniculate and tritiate cartilages.

Arteries

The main arterial supply of the head and neck is derived from branches of the carotid and subclavian arteries ( Figs 33.1 – 33.2 ). Branches of the internal carotid and vertebral arteries anastomose in the circle of Willis within the interpeduncular cistern on the ventral aspect of the brain ( Ch. 26 ). FLOAT NOT FOUND FLOAT NOT FOUND

Carotid system

The cervical portion of the common carotid artery is similar on both sides. Each lies within the carotid sheath of deep cervical fascia, together with the internal jugular vein and vagus nerve. In the lower part of the neck, the arteries are separated by a narrow gap that contains the trachea, and higher up they are separated by the thyroid gland, larynx and pharynx. At the level of the upper border of the thyroid cartilage (C4), the common carotid artery bifurcates into external and internal carotid arteries: there can be significant variation on one or both sides of the neck. The external carotid artery passes upwards on either side of the neck, inclined at first slightly forwards and then backwards and a little laterally. It usually gives off the ascending pharyngeal, superior thyroid, lingual, facial, occipital and posterior auricular arteries, and then enters the parotid salivary gland where it divides into its terminal branches, the superficial temporal and maxillary arteries. The branches of the external carotid artery supply the face, scalp, tongue, upper and lower teeth and gingivae, palatine tonsil, paranasal sinuses and nasopharyngeal tube, external and middle ears, pharynx, larynx and superior pole of the thyroid gland. They also anastomose with branches of the internal carotid arteries on the scalp, forehead and face, in the orbit, nasopharynx and nasal cavity, and with branches of the subclavian artery in the pharynx, larynx and thyroid glands.

The internal carotid artery supplies most of the ipsilateral cerebral hemisphere, the eye and accessory organs, the forehead and, in part, the external nose, nasal cavity and paranasal sinuses. It passes up the neck anterior to the transverse processes of the upper three cervical vertebrae and enters the cranial cavity via the carotid canal in the petrous part of the temporal bone. The artery has no branches in the neck and so is easily distinguishable from the external carotid artery, should the latter require ligation, e.g. to control haemorrhage from a penetrating injury to the neck ( ).

Veins

The veins of the neck lie superficial or deep to the deep investing fascia. Superficial veins ultimately drain into either the external, anterior or posterior external jugular veins; they drain a much smaller volume of tissue than the deep veins. Deep veins tend to drain into either the internal jugular vein or the subclavian vein. The internal jugular vein drains blood from the skull, brain, superficial face and much of the neck. It descends in the neck within the carotid sheath and unites with the subclavian vein behind the sternal end of the clavicle to form the brachiocephalic vein ( Figs 33.3 – 33.4 ). At its junction with the internal jugular vein, the left subclavian vein usually receives the thoracic duct, and the right subclavian vein receives the right lymphatic duct.

Lymphatic drainage

Lymph nodes in the head and neck are arranged in two horizontal rings and two vertical chains on either side of the neck ( Fig. 33.5 ). The outer, superficial, ring consists of the occipital, preauricular (parotid), submandibular and submental nodes, and the inner, deep, ring is formed by clumps of mucosa-associated lymphoid tissue (MALT) located primarily in the nasopharynx and oropharynx (Waldeyer's ring). The vertical chain consists of superior and inferior groups of nodes related to the carotid sheath. All lymph vessels of the head and neck drain into the deep cervical nodes, either directly from the tissues or indirectly via nodes in outlying groups. Lymph is returned to the systemic venous circulation via either the right lymphatic duct or the thoracic duct.

Cranial nerves

There are 12 pairs of cranial nerves. They are individually named and numbered (using Roman numerals) in a rostrocaudal sequence (see Table 24.1 ). Some are functionally mixed, others are either purely motor or purely sensory, and some also carry pre- or postganglionic parasympathetic fibres that are secretomotor to the salivary and lacrimal glands or motor to the smooth muscle within the eyeball and orbit. Branches of the oculomotor, trochlear, trigeminal, abducens, facial, glossopharyngeal, vagus, accessory and hypoglossal nerves supply muscle groups within the eyeball, face, neck, pharynx, larynx and tongue. Branches of the trigeminal, glossopharyngeal and vagus nerves transmit general sensory information from the skin of the face and part of the scalp; the epithelium lining the oral and nasal cavities, the paranasal sinuses, middle ear, pharynx and larynx, and the dorsal surface of the tongue and the cornea; the intracranial meninges; and the periosteum and bones of the skull. Branches of the trigeminal nerve innervate the temporomandibular joint. The olfactory, optic, trigeminal, facial, vestibulocochlear and vagus nerves contain axons that transmit the special sensations of olfaction, vision, hearing, balance and taste. The olfactory nerve is the only sensory cranial nerve that projects directly to the cerebral cortex rather than indirectly via the thalamus ( Ch. 32 ). The optic nerve terminates in the thalamus ( Ch. 30 ). The other ten pairs of cranial nerves are attached to the brainstem or, in the case of the (spinal) accessory nerve, to the upper cervical spinal cord; their component fibres arise from or terminate in named cranial nerve nuclei. The cranial nerves pass through named foramina in the skull, often with named vessels.

With one exception, all the cranial nerves are confined to the head and neck. The exception is the vagus, which travels through the neck and thorax, and enters the abdominal cavity by passing through the respiratory diaphragm with the oesophagus.