Head and neck

13 Head and neck




ANATOMY



ANATOMY OF THE NECK


A thorough knowledge of anatomy is required to treat surgical conditions affecting the neck. For descriptive purposes the neck is divided into various triangles (Fig. 13.1). The sternocleidomastoid (SCM) divides it into two large triangles, the anterior triangle between the SCM and the midline and the posterior triangle between it and the trapezius.




Surface anatomy of the neck


The following can be felt in the midline from above downwards (Fig. 13.2):









The lower border of the cricoid is an important level in the neck and it corresponds to:







Each SCM can be tensed and tested by turning the head against resistance to the opposite side.


The pulsation of the common carotid artery can be felt at the anterior border of SCM at the lower border of the cricoid cartilage (C6 level). The common carotid artery usually bifurcates at the upper border of the thyroid cartilage.


The lower end of the internal jugular vein is located in the gap between the sternal and the clavicular heads of the sternocleidomastoid muscle.




Deep fascia of the neck


The neck has distinct fascial layers which facilitate block dissection in the treatment of metastatic tumours. The layers of the fascia form lines of cleavage during operative dissection and also to a certain extent limits the spread of pus during infection.


There are three distinct layers (Fig. 13.3):






The investing layer is the outer of the three, arising from the ligamentum nuchae and the spines of the cervical vertebrae to completely surround the neck. It splits to enclose the trapezius and the SCM and between these two forms the roof of the posterior triangle and also contributes to the fascial capsules of the parotid gland and the submandibular glands. Above it is attached to the external occipital protuberance, mastoid process, and the zygomatic arch and the mandible. Below, it is attached to the manubrium sterni, the clavicle, the acromion and the spine of the scapula.


The prevertebral fascia is anterior to the vertebral column, the prevertebral muscles and the scalene muscles. It prolongs into the axilla as the axillary sheath enclosing the brachial plexus and the subclavian artery. In an axillary block of the brachial plexus the local anaesthetic is introduced into the axillary sheath. In an interscalene block the plane deep to the fascia is infiltrated as it contains the roots and trunks of the brachial plexus.


The pretracheal fascia splits into an anterior layer that encloses the infrahyoid (strap) muscles and a posterior layer which forms the fascial capsule of the thyroid gland. The fascia extends into the mediastinium and merges with the pericardium. Laterally it blends with the investing layer deep to the SCM. It also contributes to the carotid sheath.



THE ANTERIOR TRIANGLE


The anterior triangle is bounded by the anterior border of the SCM, the midline and inferior margin of the mandible. It is subdivided into four smaller triangles: submental, submandibular, carotid, and muscular (Fig. 13.1). The anterior triangle contains among other structures the thyroid gland, the submandibular gland, the carotid sheath, the deep cervical group of lymph nodes, and the supra and infra hyoid groups of muscles.




Blood vessels in the Anterior Triangle



Carotid arteries


The right common carotid artery is a branch of the branchiocephalic trunk; the left common carotid is a branch of the arch of the aorta. The common carotid artery divides into the external and the internal carotid arteries at the upper border of the thyroid cartilage. The bifurcation can be at a higher level, a point worth remembering to avoid ligation of the common carotid instead of the external carotid.


The common carotid artery is crossed at the level of the 6th cervical vertebra by the omohyoid muscle (Fig. 13.4). Above this level the artery is superficial and its pulsation can easily be felt whereas below, the artery is covered by the infrahyoid muscles and the SCM. The artery is enclosed in the carotid sheath with the internal jugular vein lateral to it and the vagus nerve between the artery and the vein at a deeper plane. The internal carotid artery passes vertically upwards as a continuation of the common carotid without giving any branches in the neck. The artery which is also enclosed in the carotid sheath is separated from the external carotid by (Fig. 13.5):







It is accompanied by a plexus of sympathetic nerves. At the base of the skull the artery enters the carotid canal. The intracranial part supplies the eye and the brain.


The external carotid artery extends from the point of bifurcation of the common carotid to a point midway between the angle of mandible and the mastoid process. The upper part of the artery enters the parotid gland where it divides into its two terminal branches: the maxillary artery and the superficial temporal artery. At its commencement the artery is anteromedial to the internal and can be distinguished from the internal by the presence of branches (the internal carotid has no branches in the neck). The branches of the external carotid artery are (Fig. 13.6):











The superior thyroid artery arising at the commencement of the external carotid is closely related to the external laryngeal nerve. The nerve should be identified and separated before ligating the artery during thyroid surgery.


The external carotid artery may have to be ligated to control bleeding from one of its inaccessible branches. However, ligation will not eliminate blood flow through it because of the anastomoses of the branches of the arteries of the two sides.





POSTERIOR TRIANGLE



The boundaries are









The skin over the triangle has platysma only in its anterior part. Its absence and hence relatively lower vascularity makes development of skin flaps in the posterior part more difficult.


The posterior triangle contains the subclavian artery (3rd part), transverse cervical artery, suprascapular artery and the occipital artery.


The external jugular vein courses in the superficial fascia obliquely, pierces the deep fascia just above the clavicle and drains into the subclavian vein. Dissection of the lower part of the triangle may cause troublesome bleeding from this vein. The spinal accessory nerve is the most important structure in the posterior triangle. It exits from the jugular foramen, passes through the deep part of the sternocleidomastoid and enters the posterior triangle where it lies fairly superficially embedded in the deep fascia along the roof. It then enters the under surface of the trapezius. The nerve supplies the sternocleidomastoid and the trapezius. The accessory nerve can be damaged during biopsy of lymph nodes in the posterior triangle. This will paralyse the trapezius resulting in inability to raise the arm above the level of the shoulder as well as inability to shrug the shoulder.




MANDIBLE


The mandible, or the lower jaw, consists of a horizontal body bearing the alveolar process and the lower teeth, and a vertically orientated ramus. The junction between the body and the ramus is the angle of the mandible. The upper part of the ramus divides into an anterior coronoid process and a posterior condyloid process which bears the head and neck of the mandible (Fig. 13.7). The head articulates with the mandibular fossa at the base of the skull to formthe temporo-mandibular joint. The neck has a depression, the pterygoid fovea, in its upper part for the insertion of the lateral pterygoid muscle. The coronoid process receives the attachment of the temporalismuscle.




Medial surface


On the medial aspect of the ramus is the mandibular foramen (Fig. 13.8). This is guarded anteriorly by a projecting process called the lingula to which the sphenomandibular ligament is attached. The inferior alveolar (dental) nerve enters the mandibular foramen and traverses the body within the mandibular canal. It divides into the mental nerve and the incisive nerve. The incisive nerve which supplies of the incisors and canine teeth runs beyond the mental foramen within the body in the incisive canal. The trunk of the inferior alveolar nerve supplies the premolars and the molars.



A small groove runs inferiorly and forward from the mandibular foramen. This is the mylohyoid groove and is produced by the nerve to mylohyoid which supplies the mylohyoid and the anterior belly of the digastric muscles. Above the groove is a prominent ridge, the mylohyoid line for the attachment of the mylohyoid muscle. The muscle extends from the level of the last molar tooth to the midline. The two mylohyoids which form the floor of the mouth separate the oral cavity from the neck. The slight depression on the bone below the mylohyoid line is the submandibular fossa where the superficial part of the submandibular gland is located. The deep part of the gland and the sublingual gland lie above the mylohyoid line in the oral cavity. This part of the mandible is lined by the mucous membrane of the mouth.


The rough area on the medial surface of the angle of the mandible is for the attachment of the medial pterygoid muscle.


Anteriorly in the midline are two pairs of irregular elevations, the genial tubercles or mental tubercles. The superior pair give attachments to the genioglossi and the inferior to the geniohyoids.





MUSCLES OF MASTICATION


There are four pairs of muscles in this group attaching the mandible to the base of the skull:














TONGUE


The tongue lies on the floor of the mouth and extends into the anterior wall of the oropharynx. It is a mass of striated muscles covered by mucous membrane. Its mobility is essential for mastication, swallowing and speech. It is derived from a variety of embyonic sources. The anterior two-thirds of the mucosa is developed from the first branchial arch and the posterior third from the third. Both intrinsic and extrinsic muscles are from the occipital myotomes.









FLOOR OF THE MOUTH


The floor of the mouth separating the oral cavity from the neck is formed by the mylohyoid diaphragm formed by the fusion of the mylohyoid muscles of both sides along the midline raphe Above the mylohyoid is the mouth and below is the neck. The mylohyoids are reinforced superiorly by the two geniohyoids. The anterior part of the tongue rests on the mucosa covering the floor of the mouth. In the midline, the frenulum of the tongue is seen on the floor connecting the tongue to the mandible. On either side of the frenulum is the sublingual papilla on which the submandibular gland duct opens (Fig. 13.10). Lateral to this is the sublingual fold produced by the sublingual gland.


More posteriorly between the mylohyoid and the tongue lies the hyoglossus muscle which in fact is the side wall of the tongue. A number of important structures in the floor of the mouth lie on the hyoglossus. These from above downwards are:





The deep part of the submandibular gland and the submandibular duct are described on page 72.


The lingual nerve, a branch of the mandibular division of the trigeminal nerve, runs forward above the mylohyoid. It gives off a gingival branch which supplies the whole of the lingual gingiva and the mucous membrane of the floor of the mouth. The lingual nerve winds round the submandibular duct (page 72) before getting distributed to the mucosa of the anterior two-thirds of the tongue. The submandibular ganglion is suspended from the lingual nerve as it lies on the hyoglossus. The preganglionic fibres in the chorda tympani synapse in this ganglion. Before reaching the floor of the mouth the lingual nerve lies against the periosteum of the alveolar process closely related to the 3rd molar tooth. The nerve can be damaged here during dental extraction.


The hypoglossal nerve descends between the internal jugular vein and the internal carotid artery, giving branches to thyrohyoid and geniohyoid muscles. It supplies the superior limb of the ansa cervicalis (C1) to innervate the infrahyoid muscles. It reaches the surface of the hyoglossus by passing deep to the posterior belly of the digastric. On the hyoglossus it breaks up into branches to supply all the muscles (both extrinsic and intrinsic) of the tongue except the palatoglossus. Paralysis of the hypoglossal nerve is manifested as fibrillation of the tongue as well as wasting of the muscles. The latter will show the mucosa loose on the paralysed side.



THE PALATE


The roof of the mouth is the palate. The anterior two-thirds is bony, forming the hard palate and the posterior third, the soft palate, is muscular. The midline projection of the soft palate backwards is the uvula. If the subject says ‘aah’ the soft palate will move upwards. The palatine process of the maxilla and the horizontal plate of the palatine bones form the hard palate. The tensor palatini, the levator palatini, the musculus uvuli, the palatoglossus and the palato-pharyngeus form the muscular core of the soft palate. The tensor palatini winds round the pterygoid hamulus of the medial pterygoid plate to enter the cavity of the pharynx and its tendon spreads out to become the palatine aponeurosis to be attached to the posterior aspect of the hard palate. The levator palatini takes origin from the base of the skull inside the pharynx and is inserted to the palatine aponeurosis. The other palatine muscles merge with the aponeurosis. Both the tensor and the levator palatini in their upper part are attached to the cartilaginous part of the Eustachian (auditory) tube. Their contraction opens the tube to transmit air from the pharynx to the middle ear. Children with cleft palate may develop deafness as this mechanism is often affected.


The mucosa of the palate has stratified squamous epithelium on the oral surface and ciliated columnar epithelium on the surface facing the nasal cavity. The sensory nerve supply of the palate is by branches from the maxillary nerve and the motor supply is by the cranial part of the accessory nerve transmitted through the vagus as its pharyngeal branch.



SALIVARY GLANDS



Parotid gland


This serous salivary gland has a complex shape, irregular surfaces and important relations. An anatomy teacher told his students that during the Creation of Man the Creator poured ‘liquid parotid tissue’ into the area between the mastoid process and the ramus of the mandible, the liquid trickled into all the crevices in this region and solidified around a number of important structures. The story emphasises the complex configuration and relations of the gland which will no doubt be appreciated by a surgeon doing a total parotidectomy.


The parotid gland lies between the mastoid process and the sternocleidomastoid posteriorly, and the ramus of the mandible, which it clasps anteriorly (Fig. 13.13).



The upper pole of the gland is a small concave surface and it adheres to the cartilaginous part of the auditory tube and it is wedged between the latter and the capsule of the temporomandibular joint.


The lower pole extends below and behind the angle of the mandible into the neck on to the SCM and the posterior belly of the digastric.


The parotid gland is enclosed in a tough capsule derived from the investing layer of the deep fascia. Inflammation of this gland produces pain as the gland swells within the unyielding capsule.


The parotid duct or Stensen’s duct emerges from the anterior border of the gland, lies over the masseter, turns medially to pierce the buccinator to enter the oral cavity at the level of the upper second molar tooth. It lies between the muscle and mucous membrane for a short distance before piercing it and the valvular flap thus produced prevents inflation of the gland when the intra-oral pressure is raised. The duct is palpable over the masseter when the jaw is clenched. It lies along a line joining the tragus of the ear and the philtrum of the upper lip. Classical descriptions attribute three surfaces to the gland:









Structures passing through the parotid gland


The external carotid artery, the retromandibular vein and the facial nerve pass through the parotid gland (Fig. 13.14).



The external carotid artery enters the posteromedial surface inferiorly and divides within the gland into its terminal branches, the maxillary and the superficial temporal arteries. The terminal branches leave the anteromedial surface. The retromandibular vein, which emerges from the posteromedial surface, is formed within the gland by the union of the maxillary and the superficial temporal veins which enter the gland on its anteromedial surface.


The facial nerve leaves the base of the skull through the stylomastoid foramen. The main trunk of the nerve is located in the triangle formed by the mastoid, the angle of the mandible and the cartilaginous part of the external auditory meatus. During parotidectomy, the trunk of the nerve is approached along a plane in front of the anterior margin of the cartilage. The cartilage in this region has a small projection pointing towards the nerve.


The stylomastoid branch of the posterior auricular artery is superficial to the nerve and is also a guide to its proximity.


Before entering the gland the following three branches are given off from the nerve:





The facial nerve enters the posteromedial surface of the parotid gland about 1 cm after emerging from the skull. It then passes forward in the gland as the most superficial of the three embedded structures. (The external carotid artery being the deepest.) Inside the gland the nerve usually divides into an upper temporofacial division having a vertical course and a cervicofacial division which is more horizontal. These two further divide to form the five terminal branches:







There is considerable variation in the pattern of the branching inside the gland. There are also a number of communicating rami between the branches.


The concept of a superficial and deep lobes for the parotid separated by the facial nerve is controversial as these lobes are not well defined or separated. The parotid is a common site for salivary gland tumours. Parotidectomy requires precise identification and dissection of the facial nerve and hence a precise knowledge of anatomy of the gland is essential to avoid injury to the nerve.



Submandibular gland


The submandibular gland (Fig. 13.14) and the submandibular group of lymph nodes fill the submandibular triangle which is bounded by the anterior and posterior bellies of the digastric muscle and the lower border of the mandible. The gland also extends upward deep to the mandible. Differentiating an enlargement of the gland from that of the lymph nodes can be difficult.


The superficial surface of the gland is covered by the skin, platysma and the investing layer of deep fascia and is crossed by the facial vein, the cervical branch of the facial nerve and also often by the marginal mandibular branch of the facial nerve. The marginal mandibular branch lies deep to the platysma and is one of the most important relations of the gland. This branch which supplies the depressor anguli oris and the depressor labii inferioris is liable to injury during surgery of the submandibular region. Injury of the nerve can result in facial asymmetry and occasional dribling. Skin incisions in the submandibular region are made about 4 cms below the mandible to avoid injury to the marginal mandibular branch.


Each submandibular gland has a larger, superficial part and a smaller, deep part. The two are separated by the mylohyoid muscle. The two parts, however, are continuous with each other posteriorly and the concavity thus formed is occupied by the free posterior border of the mylohyoid muscle.





Submandibular duct


The duct of the submandibular gland (Wharton’s duct) starts in the superficial part, running posteriorly and superiorly to reach the deep part. Here it turns forward and medially and emerges on to the surface of the hyoglossus muscle. It runs forward deep to the mucosa of the floor of the mouth between the mucosa and the sublingual gland and the geniohyoid muscle to open into the floor of the mouth on either side of the frenulum of the tongue. The duct, on the floor of the mouth is closely related to the lingual nerve. As it goes forward it crosses medial to the nerve to lie above the nerve and then crosses back, this time lateral to it to reach a position once again below the nerve (Fig. 13.16).



Four nerves are closely related to the submandibular glands and hence are vulnerable during its removal. marginal mandibular branch of the facial nerve may be bruised during skin incision. The nerve to mylohyoid is closely related to the superficial part of the gland. The lingual nerve can be damaged during ligation of the submandibular duct. The hypoglossal nerve is related to the deep part of the gland.




PHARYNX


The pharynx is a muscular tube attached above to the base of the skull and extends below up to the sixth cervical vertebra where it continues down as the oesophagus. It has three parts:







Oropharynx and the anatomy of the tonsil


The most important structure in the oropharynx is the palatine tonsil or the tonsil. It lies in the tonsillar fossa bounded by the anterior and posterior pillars of the fauces. The anterior pillar is the palatoglossal arch produced by the palatoglossus muscle and the posterior pillar is the palatopharyngeal arch by the palatopharyngeus muscle. The superior constrictor forms the floor of the tonsillar fossa. Pharyngobasilar fascia lining the inner surface of the constrictor forms the capsule of the tonsil and lies between the tonsil and the muscle. The capsule is normally separated from the muscle by loose areolar tissue.


The tonsil is an accumulation of lymphoid tissue. Its oral surface is lined by mucous membrane having stratified squamous epithelium. Tonsillar crypts are clefts on the inner surface and these too are lined by the mucosa (these crypts are not present in adenoids which are lined by ciliated columnar epithelium).






Muscles of the pharynx


The main muscles of the pharynx are the three fan-shaped constrictor muscles:





These are reinforced by much smaller longitudinal muscles:





Each constrictor muscle starts from a limited origin anteriorly and broadens out laterally and posteriorly to insert into a posterior midline raphe – the pharyngeal raphe. Each constrictor overlaps the one above posteriorly. There are gaps laterally. The gap between the inferior and middle are occupied by the thyrohyoid ligament and associated structures. The stylopharyngeus muscle accompanied by the glossopharyngeal nerve enters the pharynx through the gap between the middle and superior constrictors. The gap between the upper border of the superior constrictor and the base of the skull is bridged by the thick pharyngobasilar fascia. The Eustachian tube enters the pharynx through this gap.


Anteriorly the superior constrictor is attached to the pterygomandibular raphe and the middle constrictor to the greater horn of the hyoid bone. The inferior constrictor has two parts. The thyropharyngeus part of the inferior constrictor is fan-shaped like the other constrictors and is attached to the lamina of the thyroid cartilage. The cricopharyngeus part of the inferior constrictor is circular and acts like a sphincter. The weakest area of the pharyngeal wall is the gap between the thyropharyngeus and the cricopharyngeus posteriorly in the midline. This is the Killian’s dehiscence, a common site for pharyngeal diverticulum (pouch).




LARYNX


The larynx is an integral part of the respiratory tract and is the organ of voice production. It also plays an essential role in the swallowing mechanism. It is held open by a series of cartilages on its wall.



Cartilages


There are five major cartilages:









Dec 12, 2016 | Posted by in GENERAL & FAMILY MEDICINE | Comments Off on Head and neck
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