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p. 705. The head and necklocked

  • Leslie Klenerman

Abstract

‘The head and neck’ describes the anatomy of the jaw, eye and orbit, nose, pharynx, larynx, and ear, and considers some of the evolutionary processes in their development. Vision is said to be the dominant sense in humans, and the structure of the eye and the surrounding orbit is complex. The air passages of the nasal cavity lead through openings at the back of the nose, the choanae, into the pharynx. The pathways for food and air diverge in the lower pharynx (oropharynx), continuing as the larynx and oesophagus, respectively. The ear has undergone more gross evolutionary transformation in vertebrate history than the other sense organs.

One of the challenges of studying how the human head evolved and how it develops and functions is its sheer complexity.

Daniel E. Lieberman, Professor of Evolutionary Biology, Harvard University

It is striking that during our embryological development, at about the fourth week, a set of pharyngeal (branchial) arches, separated by pouches, develops. Each pair of arches grows like a collar around the foregut, eventually merging at the midline of the embryo. These arches are thought to be homologous with the gill apparatus of the fish, but in humans they progress to an entirely different fate. It helps the understanding of the anatomy of the neck to consider derivatives from them. Ultimately, the first arch tissues form the upper and lower jaws, two tiny ear bones (the incus and malleus) and the vessels and muscles that supply them. Their nerve supply is via the mandibular division of the trigeminal. The facial nerve (VII) is the nerve of the second arch and all structures that it supplies, which are predominately the muscles for facial expression. Similarly the nerve of the third arch is the glossopharyngeal (IX). The mucous membrane and glands of the back of the tongue are derived from the third arch. The nerves to the fourth and sixth arches are both branches of the vagus (X)—the superior laryngeal and recurrent laryngeal, p. 71respectively—which supply the critical functions of the larynx and therefore speech. The fourth and sixth arches become the cartilages of the larynx, while the small fifth arch disappears.

Above these arches, the head is made up of a series of compartments which are the cranial cavity (already described in Chapter 4), the lower jaw (mandible), the orbits, the nasal cavities, and the oral cavity. The lower jaw is shaped like a horseshoe. Each half is L-shaped, consisting of two oblong parts. The horizontal parts of the two sides fuse in the midline during the second year to form the body of the jaw (although in most mammals they remain paired). The alveolar process or upper part of the body of the jaw carries eight teeth in the adult. The rear portion of the jawbone (or ramus, which is Latin for oar) is an oblong, nearly vertical, flattened plate. It is surmounted by two processes, the head and the coronoid, which are separated from each other by a U-shaped notch: the mandibular notch. The mandibular canal starts at the mandibular foramen and runs through the bone—this narrow structure conveys vessels and nerves from the mandibular division of the trigeminal nerve to the teeth, thus carrying the pain of signals of toothache (Figure 23). The common crowding of human teeth—especially ‘wisdom’ teeth (third molars), which erupt last—is traceable to evolutionary shortening of our jaw. With easily digestible food available our teeth no longer had to act as heavy-duty food processors and so with the relentless economy of evolution they have slowly shrunk and our jaws have contracted for the same reason.

For movement of the jaw, a joint exists (temporomandibular joint) between the head of mandible and the temporal bone. The strength of this joint depends mainly on the bony conformation and on muscles. A fibrocartilaginous disc caps the head of the mandible and projects forward, dividing the joint cavity into an upper and lower compartment. Movement can occur between the head and disc, and both can move together on the skull, providing a wide range of motion. The heads of the mandible can be palpated by passing a finger into your ear canal and then p. 72

23. The mandible.

opening and closing your mouth, or wiggling the jaw from side to side. The various movements of the jaw are produced by the coordinated activity of the four ‘muscles of mastication’, medial and lateral pterygoids, temporalis, and the powerful masseter that clenches the teeth.

The eye and orbit

If any one sense can be said to be dominant in humans it must be vision. Primates have been especially visually oriented since our divergence from other mammals. The eye is thought to have developed originally from a simple patch of light-sensitive pigment cells in invertebrates, and such structures are still present in flat worms. The cavities of the orbits are approximately p. 73

24. The positions of the orbits.

pyramidal in shape. The medial walls are parallel, separated by the nasal cavities, and as a result the axes of the bony orbits lie at 45 degrees to each other. This is of importance for understanding the actions of the six extra-ocular muscles controlling eye movements (Figure 24). The apex is at the optic canal, the site of entry of the optic nerve. In mammals lower than primates there is no bony lateral wall, which is unexpected because in man this wall is strong for transmitting forces from the molar teeth upwards. In many mammals (e.g. the cat or pig) even the lateral part of the orbital margin is missing, although, the lines of force transmission in these animals are distinct from those in humans. The eyeball occupies the front half of the cavity, while muscles and fat largely fill the back half. There are four straight and two oblique muscles which act in a tightly regulated way between the two eyes to provide co-ordinated binocular vision (if this fails, even slightly, this can lead to the phenomenon of ‘double vision’).

p. 74The eyeball contains the light-sensitive retina and like a camera it is provided with a lens system for focusing images and the ability to control the amount of light entering the eye via the iris and pupil. The colour of the eye, blue or brown, is due to the colour of the iris, which is a thin circular muscle controlling the size of the pupil. Most babies of European descent are born with blue eyes. With the passage of time, pigment is deposited, and varying with the amount laid down the colour changes. If little is deposited, the eye remains blue or grey, while at the opposite extreme the eye becomes brown. In Africans the iris is pigmented at birth, and hence in the newborn the iris is not blue.

The wall of the eyeball has three layers. The outer coat is fibrous and consists of the white sclera and translucent cornea. A vascular coat, the black choroid, intervenes between the sclera and innermost nervous layer (the retina). It is black to prevent internal reflections. The retina has several layers and oddly is sensitive to light in the outer layer (this is not the case in most invertebrates and may be considered ‘inside out’ because the photoreceptors lie behind its ganglion cells). Vision is most acute where rays of light come to focus on the retina at the posterior pole. This part of the retina is the yellow spot or macula. There are two kinds of photoreceptor cells, rods and cones. The centre of the macula, the fovea, is a shallow pit with a particularly high density of cones from which blood vessels are diverted away, thereby decreasing light refraction. The optic nerve pierces the sclera 3 millimetres (mm) to the medial or nasal side of the posterior pole at the optic disc which is the ‘blind spot’. The optic disc is clearly visible with an opthalmoscope and is a valuable part of the clinical examination (Figure 25). The appearance of the optic disc on examination varies in different disease processes and can also provide clues as to what is happening in the brain.

The lacrimal gland, situated in the upper lateral part of the orbit secretes tears through a series of ducts into the upper part of the conjunctiva. The conjunctiva is the delicate mucous p. 75

25. Sagittal section of the eye.

membrane lining the inner surface of the eyelids from which it is reflected over the anterior part of the sclera to the cornea. Tears moisten the front of the eye, preventing friction between the eyeball and lids and drying of the corneal epithelium. Tears are drained away through small holes seen near the medial margin of each eyelid. They are collected in the lacrimal sac situated in a small depression on the medial surface of the orbit. This in turn drains via the nasolacrimal duct into the front of the nose, which results in snuffling in the tearful.

When things go wrong

From the vestigial remnants of the second branchial cleft, a cyst (a swelling consisting of a collection of fluid in a sac which is lined by epithelium) may develop. This usually is seen along the line of the sternocleidomastoid muscle, the large muscle clearly seen on either side of the neck. The treatment is by excision.

p. 76The retina is at particular risk in relation to injury to the eye. The outer layer of the retina is firmly attached to the choroid but only loosely attached to the inner neural layer, which is largely held in place by the vitreous humour on the inside of the eyeball. High impact forces from blows to the head can sometimes produce enough shearing force to separate the two layers and tear the retina. This is particularly the case in the severely nearsighted because their eyes are longer than average in the anteroposterior direction, which stretches the retina and causes it to be more fragile.

With aging, the lens may become opaque and lose the capacity to transmit light resulting in a cataract. Nowadays removal of cataracts and replacement with a prosthetic lens (silicone or acrylic) is one of the most frequent surgical operations.

The floor of the orbit is its weakest wall, and in blunt trauma such as fist injuries it is often fractured without fractures of the other walls. There may be air in the tissues because the orbital floor is the roof of the maxillary (upper jaw) air sinus (see the next section, which discusses nasal passages). If there is little displacement the fracture heals spontaneously.

The nose

The nasal passages represent a filter through which air must pass en route to the lungs. Beginning in the nasal cavity the air is modified to make it more tolerable to the body. Warming requires a large surface so the walls of the nose are expanded by three conchae, bony scrolls attached to the lateral wall. The superior concha and corresponding region of the septum are covered with olfactory epithelium in which the smell receptors are embedded. Compared with that of other mammals, this is a small area, which corresponds with the reduced emphasis on the sense of smell in man. Both the number and complexity of the conchae (called turbinals in other mammals) are greater in nonprimates.

p. 77The nasal cavity connects with the paranasal sinuses. These sinuses are air spaces in several of the facial bones: frontal, ethmoid, sphenoid, and maxilla (the upper jaw). They are bilateral but not symmetrical. Each of the sinuses is lined with ciliated columnar mucous membrane and drains to an opening under cover of the superior or middle concha. The maxillary sinus drains upwards, via an ostium near its roof, a remnant of our time as quadrupeds. The reason for the presence of the paranasal sinuses has been a controversial subject since the time of Galen. Their functions are not clearly known. Several suggestions for the sinuses have been proposed, such as for reduction in weight of the facial bones, to give resonance to the voice, and for thermal insulation for the brain.

Because they represent ‘dead ends’ in air circulation, sinuses are frequently sites of infection and congestion (Figures 26 and 27).

26. Sagittal section of head showing nose, pharynx, and larynx.

p. 78

27. Coronal section of face showing orbits, interior of nose, and maxillary sinuses.

The nasal septum separates the right and left nasal cavities. It is made of two bones (ethmoid and vomer) and hyaline cartilage. The septum may not be quite straight and this usually occurs at the junction of bone with cartilage. Just inside the nostrils on the anteroinferior part of the nasal septum is an area where most nose bleeds start, which is called Little’s area; it is a site of a confluence of vessels.

Pharynx

The pharynx is a fibromuscular tube that extends from the base of the skull to the lower border of the cricoid cartilage where at the level of the sixth cervical vertebra it is continuous with the oesophagus. The muscles involved are three constrictors: superior, middle, and inferior. The inferior constrictor overlaps the middle p. 79and the middle overlaps the superior in telescopic fashion. They are joined together by a posterior midline raphe (or seam). Cricopharyngeus, the lowest part of the inferior constrictor acts as the sphincter (controlling muscle) for the oesophagus.

The air passages of the nasal cavity lead through the openings at the back of the nose, the choanae, into the pharynx. The soft palate acts as a valve to separate the nasal cavity from the oral cavity and to prevent the upward movement of food during swallowing. The pathways for food and air diverge in the lower pharynx (oropharynx), continuing as the larynx and oesophagus, respectively. The epiglottis forms a barrier that deflects food away from the entrance to the larynx during swallowing to prevent choking. In newborns, the larynx is high in the neck and the epiglottis is above the level of the soft palate. Babies can therefore suckle and breathe at the same time. During the second year of life the larynx descends into the low cervical position characteristic of adults.

The pharynx can be described in three parts from top to bottom: nasopharynx, oropharynx, and laryngopharynx. The nasopharynx lies above the soft palate, which cuts it off from the rest of the pharynx during swallowing and thus prevents regurgitation of food through the nose. Two important structures are in this compartment. The nasopharyngeal tonsillar tissue, commonly known as ‘the adenoids’, is a collection of lymphoid tissue beneath the epithelium of the roof and posterior wall of this region. It forms part of a continuous lymphoid ring with our tonsils (formally described as palatine tonsils) and lymphoid nodules on the back of the tongue. This arrangement produces a ring of lymphoid tissue known as Waldeyer’s Ring to act as a guard for the entrance to larynx and oesophagus, protecting against invading pathogens entering from the mouth and nose. The orifice of the pharyngotympanic or auditory tube (Eustachian canal) lies on the side-wall of the nasopharynx level with the floor of the p. 80nose. This has the important function of allowing equalization of pressure between the middle and outer ears so as to allow our ear drums to function normally (this is described later in the section on the ear).

The oropharynx is continuous with the oral cavity (mouth). It extends from the uvula of the soft palate (visible in the midline as a structure which hangs down at the back of the throat) above to the tip of the epiglottis below. Its most important contents are the palatine tonsils (our common tonsils). The laryngopharynx extends from the level of the tip of the epiglottis to where the pharynx terminates and the oesophagus begins. The inlet of the larynx lies in front and there is a deep recess on either side of the larynx known as the piriform fossa in which sharp ingested foreign bodies such as fishbones may lodge. The vallecula consist of two depressions on either side of the midline at the base of the tongue, separated by a median fold of mucous membrane. This is a convenient site for the blade of a laryngoscope (an instrument used for passing a tube into the trachea during anaesthesia) as one can pull the tongue forwards.

Larynx

The larynx is a cube shaped box at the upper end of the trachea. It consists of a series of cartilages suspended from the hyoid bone (a U-shaped bone situated at the level of the chin suspended by muscles). The foundation of the laryngeal ‘skeleton’ is the cricoid cartilage, the only complete cartilaginous ring in the respitatory tract—it articulates with the slender arytenoid and flattened thyroid cartilages. The thyroid has two laminae (thin plates) whose anterior junction constitutes the laryngeal prominence—more prominent in men and known as the Adam’s Apple.

Within the larynx, the vocal folds regulate the flow of air out of the lungs and are responsible for voice. A vocal fold consists of a vocal ligament, stretching from the thyroid cartilage anteriorly p. 81

28. Sagittal section of larynx.

to the arytenoid cartilage posteriorly, and a tiny vocalis muscle, which is parallel and adherent to the ligament covered by mucous membranous continuous with the lateral wall. By movements of the cartilages, the free edges of the vocal folds can be shifted subtly medially and laterally to open and close the air passage. If the vocal folds are tensed, air passing between them causes them to vibrate. The vibrations are translated into sound waves as voice. The pitch of the voice corresponds to the degree of tension in the vocal folds (Figure 28).

The ear

The ear has undergone more gross evolutionary transformation in vertebrate history than have other sense organs. The outer ear (pinna) and its external auditory canal collect sounds and transmit them into the cranium. Once sound waves reach the end of the outer ear they are transmitted into the middle ear (an p. 82air-filled space within the petrous temporal bone, which is part of the middle cranial fossa) via the eardrum (tympanic membrane). This is a mostly taut fibrous membrane at the end of the bony external auditory canal. Like a drum, the tympanic membrane bows inwards and then deflects backwards with every wave of pressure. The membrane presses against a chain of three middle-ear ossicles: the malleus (hammer), incus (anvil), and stapes (stirrup). They are the first bones in the body to reach adult size at twenty-five weeks after conception. The ossicles function as a bent-lever amplifier. The long handle of the malleus is attached to the tympanic membrane and the stapes is pushed against the oval window, which connects the middle ear to the inner ear. For the tympanic membrane to function optimally the pressure in the middle and outer ear need to be the same. Equalization of pressure is made possible by the pharyngotympanic (Eustachian) tube, which connects the nasopharynx and the middle ear.

The final journey of a sound wave occurs in the cochlear part of the inner ear, which changes the mechanical energy of the amplified sound waves into nerve impulses that are transmitted to the brain. The cochlea (from ‘snail’) is a spiral-shaped tube of bone with several chambers. Sound waves from the middle ear travel through the fluid of the cochlea every time the stapes knocks against the membrane of the oval window. The lower the frequency, the further the wave moves up the cochlea. As a result it causes vibrations that bend sensory hair cells within the organ of Corti in the central part of the cochlea. This complex organ has a basilar membrane from which project thousands of tiny hair cells (in four rows) that contact an overlying tectorial membrane. When the basilar membrane vibrates, the hair cells bend against the tectorial membrane, stimulating the cell nuclei to send a nerve impulse up the cochlear nerve to the brain. The hair cells are tuned to different frequencies, with higher frequencies received near the oval window and lower ones at increasing p. 83

29. The external, middle, and inner ear.

distances from the oval window. There is also a round window, where bone is absent, surrounding the cochlea at the basal end of the tube. One consequence of this structural arrangement is that inward movement of the oval window displaces the fluid of the inner ear, causing the round window to bulge out slightly and deforming the cochlear partition (Figure 29).

The sense of movement is an essential component of all motor activity. Balance involves analysing sensory inputs from the eyes, skin, muscles and joints and the ear also plays a central role here. The vestibule and semicircular canals in the inner ear are the balance organs. The three interconnected semicircular canals are at right angles to each other and can respond to any head movement The vestibule, which consists of the utricle and saccule, responds mainly to the position of the head relative to gravity (static equilibrium), while the semicircular canals react to the speed and direction of head movements (dynamic equilibrium).

When things go wrong

Fractures of the skull may involve the cribriform plate which allows the passage of olfactory nerve rootlets from the olfactory bulb into the nose. This results in leakage of cerebrospinal fluid from the nostrils (rhinorrhea) because of communication with the subarachnoid space. Meningitis is a potential complication. Even blows to the head that do not cause fractures can lead to shearing of the olfactory nerve fibres as they pass through this plate, which results in loss of smell (anosmia). This can diminish the enjoyment of food, influence appetite, and cause weight loss.

Cleft palate with or without cleft lip occurs once in about 2,500 births. There is defective growth of the palatal shelves. Children with cleft palate usually show underdevelopment in the growth of the midface and frequently a reduction of the size of the mandible. There is likely to be difficulty with feeding because food may go into the nose. There is also an increased danger of infections of the middle ear, as the opening of the pharyngotympanic tube is exposed. Cleft lips are usually repaired at 3 months of age and cleft palates at 6 months. These children require careful follow-up during growth, in the form of speech therapy and supervision by ear, nose, and throat as well as orthodontic specialists.

Problems with peripheral hearing can be divided into conductive hearing losses, which involve damage to the outer or middle ear; and sensorineural hearing losses, which stem from damage to the inner ear, most typically the cochlear hair cells or the vestibulocochlear nerve itself. Conductive hearing loss can be due to occlusion of the external ear canal by wax or foreign objects, rupture of the tympanic membrane, or arthritic ossification of the middle-ear bones. In contrast, sensorsineural hearing loss is usually due to congenital or environmental insults that lead to hair cell death or damage to the vestibulocochlear nerve (VIII). In conductive hearing losses, an external hearing aid is used to boost sounds to compensate for the p. 85reduced efficiency of the conductive apparatus. The treatment for sensorineural hearing loss is more complicated and invasive. Conventional hearing aids are useless, as no amount of mechanical amplification can compensate for the inability to generate or convey a nerve impulse from the cochlea. However, if the auditory nerve is intact, an electronic device, a cochlear implant, can be used to partially restore hearing. The electrode is inserted into the cochlea through the round window and positioned along the length of the basilar membrane.