Part 5 Head and brain

Bones

Introduction

An awesome looking structure, commonly used to portray death and instil terror, the skull is in reality a complex arrangement of many individual bones which form the skeleton of the head and face. It consists of two main elements: the large, hollow cranial cavity (the walls of which enclose the brain) and the bones of the face anteroinferiorly, which complete the walls of the orbits and nasal cavity and also forms the roof of the mouth. The mandible, although a separate bone, completes the bony framework of the face.

The discovery of many fossilized vertebrate remains, particularly skull fragments and teeth, has led to a detailed study of the skull revealing a fascinating history of the evolutionary development of the human skeleton. The size and shape of the cranial, orbital and nasal cavities, the jaw and the teeth have all provided a detailed catalogue of the probable evolutionary history of vertebrates. Estimates of relative brain size from the size and shape of the cranial cavity reveal information regarding possible intelligence. The size and shape of the jaws and teeth give clues as to the type of food eaten, and indirectly provide information regarding changes in body posture with respect to adaptation to the gathering and provision of food (e.g. running and hunting). The size and direction of the orbits, together with changes in the upper cervical spine, indicate the increasing importance of the power and range of vision. This, together with the bipedal posture adopted during the same period, gives an insight into the defence, protection and manipulation of tools necessary for survival.

The cranial cavity of the human skull is relatively large, and projects anteriorly over the facial skeleton (Fig. 5.1). Its walls are composed of plates of bone arranged as two layers of compact bone enclosing a central layer of cancellous bone. These bones ossify in the membrane (intramembranous ossification) and are joined edge to edge by fibrous interlocking joints known as sutures (p. 17).

Figure 5.1 Superior and lateral views of the adult (A, B) and fetal skull (C, D).

The top and most of the sides of the skull are formed by the two large parietal bones, which articulate along their medial borders at the sagittal suture (Fig. 5.1A). Anteriorly the skull is formed by the frontal bone, which joins the two parietal bones at the coronal suture. The point where the coronal and sagittal sutures meet is termed the bregma. At birth, the region of the bregma is not ossified and appears as an easily felt diamond-shaped area of connective tissue known as the anterior fontanelle. This gap gradually decreases in size and closes about 18 months after birth.

The frontal bone forms the forehead and separates the orbital cavities from the anterior cranial fossa. It therefore forms the roof of each orbital cavity and the floor of the anterior cranial fossa.

The remaining part of each side of the cranial cavity is formed by the squamous part of the temporal bone and part of the greater wing of the sphenoid. The region where the frontal, parietal, temporal and sphenoid bones almost meet is termed the pterion; it has a small sphenoidal fontanelle which closes within 3 months of birth. Passing medially and slightly forwards from the squamous part of the temporal bone is the petrosal part of the temporal bone, which forms most of the floor of the middle cranial fossa; it contains and protects the organs of hearing and balance.

The most posterior section of the cranial cavity is formed by the occipital bone, which meets the posterior border of each parietal bone at the lambdoid suture. The meeting point of the lambdoid and sagittal sutures is termed the lambda. Again, at birth, this region is not ossified and appears as a triangular area of connective tissue known as the posterior fontanelle. This gap rapidly decreases in size and closes between the third and sixth month after birth. The occipital bone curves forwards to surround the foramen magnum and projects anteriorly as the basilar part of the occipital bone, which joins the body of the sphenoid.

The outside of the top of the skull needs little description as the parietal bones and the relevant parts of the frontal and occipital bones show few bony landmarks except for the sutures where they join. It is, however, worth noting that the skull is widest towards the back. Its roundness means that the effects of a blow to the head can be distributed and minimized. On the inner aspect of the skull, in the region of the sagittal suture, is a faint groove formed by the superior sagittal sinus as it passes posteriorly between the two layers of the dura mater as far as the internal occipital protuberance. On either side of this groove deeply pitted areas can usually be seen. They are formed by the underlying arachnoid granulations, which themselves are involved in the regulation of the flow of cerebrospinal fluid (CSF) into the systemic circulation. The base of the skull, however, shows many features and openings which allow blood vessels and nerves to enter and leave the cranial cavity.

Growth of the skull

At birth the cranial cavity is relatively large but the face is small (Fig. 5.1D), being approximately one-eighth of the whole skull compared with one-third in the adult. The teeth are not fully formed and the paranasal sinuses are rudimentary, consequently both the jaws and nasal cavities are small. The individual bones are joined by cartilage; ossification progressing with age. There is no mastoid process so the styloid process and the stylomastoid foramen are closer to the side of the head. Undue pressure applied during a forceps delivery in the region of the developing mastoid may therefore damage the facial nerve. The maxilla is shallow because it has no sinus, and consists mainly of alveolar processes and developing teeth. The nasal bones are flat, so that the infant has no bridge to the nose, which together with the absence of superciliary arches gives the forehead its prominent appearance. The orbits are relatively large and have the nasal cavity lying almost completely between them.

After birth the skull grows rapidly until the seventh year, with the greatest increase in the size of the cranial cavity occurring during the first year. During the second year, the styloid process and the stylomastoid foramen come to lie deeper as the mastoid process begins to grow. By the seventh year the orbits are almost adult-sized; the petrous part of the temporal bone, the body of the sphenoid and the foramen magnum have, however reached full-size. The jaws have enlarged in preparation for the eruption of the permanent teeth. Growth of the skull after age 7 is slower than it was before, except for during puberty when a rapid growth in all directions occurs, particularly in the frontal and facial regions accompanying the increasing size of the paranasal sinuses.

From the early twenties to middle age there is gradual fusion of the various sutures between the individual bones of the skull, beginning with the sagittal suture.

The skull viewed anteriorly

When viewed from the front (Fig. 5.2) the upper third of the skull consists of the cranial cavity and is formed by the frontal bone; this is the forehead. Parts of the parietal, sphenoid and temporal bones can also be seen in this region. Below the forehead are the two large orbital cavities, whose margins are formed by the frontal bone above, the zygomatic bone inferolaterally and the maxilla inferomedially. The walls of the orbital cavity are formed by the maxilla inferiorly, the zygomatic bone and greater wing of the sphenoid laterally, the frontal bone and the lesser wing of the sphenoid superiorly, and the ethmoid and lacrimal bones medially. Deep within the cavity can be seen the superior and inferior orbital fissures and the optic canal. Along the medial part of the superior margin is the supraorbital notch, transmitting the corresponding vessels and nerves. Towards the front of the medial wall is the opening for the nasolacrimal duct, which conveys secretions from the eye (i.e. tears) to the nose for eventual swallowing. Below the inferior orbital margin on the front of the face is the large infraorbital foramen, through which pass the infraorbital nerve and vessels onto the face.

Figure 5.2 Anterior view of the skull.

Below and medial to the orbits is the pear-shaped nasal aperture, bound almost entirely by the maxillae, with only the superior part being bound by the two nasal bones. Within the nasal cavity a more-or-less midline septum can be seen; this is formed by the vomer and the perpendicular plate of the ethmoid. Projecting medially into the cavity from its lateral walls can be seen the inferior and the middle conchae (the two superior conchae lie deeply behind the nasal bones). The inferior alveolar margin of the maxilla, supporting the teeth, forms an almost horizontal convex arch, which fits snugly with the corresponding arch of the mandible.

The skull viewed from the lateral side

The temporal bone is central to the lateral aspect of the skull (Fig. 5.1B). Above it joins the parietal bone posteriorly and the greater wing of the sphenoid anteriorly, while posteroinferiorly it joins the occipital bone. It is marked just behind and below its centre by the external acoustic meatus, above which is a bony ramus passing forwards to meet a similar backwardly projecting process from the zygomatic bone to form the zygomatic arch. Just below the posterior part of this arch the mandibular fossa can be seen. The styloid process is seen projecting downwards from deep to the external acoustic meatus.

The skull viewed inferiorly

The skeleton of the face attaches to the undersurface of the anterior half of the cranial cavity. Consequently, a view from below reveals the mandible, maxilla and palatine bones. With the mandible removed the maxilla and the palatine bones can be clearly seen (Fig. 5.3A).

Figure 5.3 Base of the skull viewed (A) inferiorly and (B) superiorly.

The anterior and lateral margins of the maxilla are marked by the sockets for the teeth, being somewhat smaller anteriorly, increasing in size as they pass posteriorly around the alveolar margin. The area between these bony margins is formed by the palatine part of the maxilla anteriorly and the horizontal plate of the palatine bone posteriorly. Projecting backwards in the midline from the posterior border of the palatine bones is the nasal spine, while laterally are the medial and lateral pterygoid plates of the sphenoid directed inferiorly, the medial with the hook-like pterygoid hamulus. At the lateral edges of the palatine bones are the greater and lesser palatine foramina transmitting the corresponding vessels and nerves. At the front of the hard palate is the incisive canal, through which pass terminal branches of the greater palatine and sphenopalatine vessels, and the nasopalatine nerves.

The central section of the undersurface of the skull is formed mainly by the sphenoid with its many bony processes and the foramina. Laterally lies the temporal bone marked by the mandibular fossa which receives the condyle of the mandible. Posterolaterally is the external acoustic meatus, while posteromedially is the styloid process of the temporal bone – a long and slender projection passing downwards and anteromedially (Fig. 5.3A). Medial to the styloid process anteriorly is the carotid canal and posteriorly the jugular foramen.

In front of the carotid canal is the foramen spinosum, separated from it by the spine of the sphenoid. Behind the external acoustic meatus is the large breast-shaped mastoid process of the temporal bone, with the stylomastoid foramen lying medially between it and the styloid process. It is here that the facial nerve (seventh cranial nerve) emerges from the skull. Centrally the large foramen magnum is unmistakable with the occipital condyles on its anterolateral borders. The hypoglossal (anterior condylar) canal lies anterior to the condyle, while posterior is the posterior condylar canal. Posterior to the foramen magnum, and being joined to it by the external occipital crest, is the external occipital protuberance. Passing laterally towards the mastoid process is the superior nuchal line; between this line and the foramen magnum are less distinct lines, the inferior nuchal lines, which also curve laterally. The area between the two nuchal lines and the foramen magnum gives attachment to the postvertebral muscles.

The floor of the cranial cavity

The floor of the cranial cavity is formed by the base of the skull, with both the right and left sides having similar features (Fig. 5.3B). The two halves are divided in the sagittal plane by the crista galli anteriorly, the body of the sphenoid centrally, and the foramen magnum and internal occipital protuberance posteriorly. The floor is further divided into anterior, middle and posterior cranial fossae by prominent ridges of bone, with each fossa lying at a different level, the posterior being at the lowest level and the anterior the highest.

Anterior cranial fossa

This lies above and in front of the middle cranial fossa (Fig. 5.3B), being separated from it by the posterior concave edge of the lesser wings of the sphenoid. It contains the lower part of the frontal lobes of the brain. The walls and most of the floor of the fossa are formed by the frontal bone, with the posterior part of the floor being formed by the lesser wings of the sphenoid. Between the two sides is a sagittally directed elongated hollow. Running centrally in this hollow is a crest known as the crista galli on either side of which is a perforated horizontal plate, the cribriform plate. It is through this plate that the olfactory nerves from the upper part of the nasal cavity pass to the olfactory bulb above the cribriform plate.

Middle cranial fossa

This lies behind and below the anterior cranial fossa (Fig. 5.3B), being formed by the temporal and sphenoid bones. It consists of a median and two lateral parts. The lateral parts contain the temporal lobes of the brain, while the raised median part is formed by the body of the sphenoid. Passing laterally from the body of the sphenoid are the greater wings of the sphenoid, which, together with the squamous part of the temporal bone, turn superiorly, forming the lateral wall of the skull in the temporal region. The floor of the middle cranial fossa is formed by part of the greater wings of the sphenoid anteriorly and the gently sloping superior surface of the petrous part of the temporal bone posteriorly. Anteriorly each greater wing of the sphenoid turns upwards forming the anterior wall of the fossa. The superior part of this wall is overlapped by the posterior edge of the anterior cranial fossa, i.e. the lesser wings of the sphenoid.

The body of the sphenoid has a smooth, hollowed depression superiorly, the pituitary (hypophyseal) fossa, in which sits the pituitary gland. Small horn-like (clinoid) processes project on either side from the front and back of the hollow. They give attachment anteriorly to a horizontal fold of the dura mater (the tentorium cerebelli) which passes between the cerebral and cerebellar hemispheres. The cavernous sinuses (p. 591), part of the intracranial venous sinus system, are formed between the two layers of the dura mater either side of the body of the sphenoid. Within the walls, or through these sinuses, pass many of the nerves destined for the orbit, as well as the internal carotid artery. The internal carotid artery emerges from the sinus anterolateral to the anterior clinoid process. Anterior to this region, between the roots of the lesser wings of the sphenoid, is the optic canal running forwards and laterally conveying the optic nerve and ophthalmic artery to the orbit. Passing transversely between the two optic canals is a groove called the sulcus chiasmatis; it does not lodge the optic chiasma. Between the greater and lesser wings of the sphenoid is a gap which becomes narrower as it passes laterally. This is the superior orbital fissure and opens directly into the back of the orbit. It transmits many structures to and from the orbit – the third, fourth, ophthalmic division of the fifth, and sixth cranial nerves, and the ophthalmic veins. Below the superior orbital fissure in the anterior wall of the middle cranial fossa close to the body of the sphenoid is the rounded foramen rotundum, through which passes the maxillary division of the fifth cranial nerve into the pterygopalatine fossa.

In the floor of the middle cranial fossa between the greater wing of the sphenoid and the petrous temporal bone is the foramen lacerum. In life, its edges are connected by fibrous tissue, which supports the internal carotid artery as it passes medially from the carotid canal to the side of the body of the sphenoid. Lateral to the foramen lacerum are two openings in the greater wing of the sphenoid. The larger, medial one is the foramen ovale, which transmits the mandibular division of the trigeminal nerve as well as the lesser petrosal nerve and small blood vessels. The more lateral opening is the foramen spinosum, through which passes the middle meningeal artery to supply the meninges of the brain.

Posterior cranial fossa

The largest and deepest of all three fossae (Fig. 5.3B); in it lodges the cerebellum. The temporal bone is seen as a hard ridge passing backwards and laterally from the body of the sphenoid. Along its superior border runs a longitudinal groove for the superior petrosal sinus.

The floor and most of the posterior wall of the fossa are formed by the concave surface of the occipital bone; a small part of the posterior wall being formed by the parietal bones. The anterolateral wall is formed by the posterior surface of the petrous part of the temporal bone, with the body of the sphenoid and the basilar part of the occipital bone forming the anterior wall.

The most obvious feature of the posterior fossa is the large oval opening (foramen magnum), slightly narrower transversely than from front to back. It transmits the spinal cord, being at the junction of the spinal cord and brainstem, as well as a number of blood vessels and the spinal part of the 11th cranial nerve. Passing upwards from the posterior margin of the foramen magnum is the internal occipital crest ending at the internal occipital protuberance. Running down towards this protuberance is the continuation of the groove of the superior sagittal sinus. As it approaches the protuberance, the groove passes to the right to run transversely around the posterolateral wall of the fossa as the groove for the transverse sinus. On reaching the petrous part of the temporal bone, the groove turns downwards and continues as an S-shaped towards the jugular foramen. This latter groove is known as the groove of the sigmoid sinus. A similar arrangement of transverse and sigmoid grooves can be observed in the left-hand side of the posterior cranial fossa. The sigmoid sinus passes through the jugular foramen to become the internal jugular vein. As it does so it is joined by the inferior petrosal sinus, which runs in the groove between the petrous temporal and the basioccipital bones towards the jugular foramen. As well as these two venous channels passing through the jugular foramen, it also transmits the ninth, tenth and eleventh cranial nerves.

Anterior to the jugular foramen is the carotid canal through which passes the internal carotid artery and its associated plexus of sympathetic nerves. In the posterior surface of the petrous part of the temporal bone is the opening of the internal auditory meatus; both the seventh and eighth cranial nerves enter this canal.

Running forwards and upwards from the anterior margin of the foramen magnum is the clivus, which has the basilar artery separating it from the brainstem. On the lateral part of the clivus, medial to the groove for the inferior petrosal sinus, runs the sixth cranial nerve before piercing the dura to run in the sinus. Above the lateral margin of the foramen magnum is the hypoglossal canal, which runs anterolaterally and transmits the 12th cranial nerve.

Palpation

Virtually the whole of the outer surface of the cranium is palpable, being either subcutaneous or lying just below a thin sheet of muscle. The top of the skull composed of parietal and frontal bones can be taken between the examiner’s hands as in giving a blessing. On the upper part of the occipital bone, the external occipital protuberance can be readily determined by the finger tips. On the temporal bone, to the side of the skull, the external acoustic meatus is visible, and behind the pinna of the ear the mastoid process can be identified becoming pointed at the level of and behind the lobe of the ear. From just above the external acoustic meatus the posterior part of the zygomatic arch can be identified running forwards to join the zygomatic bone, which presents as the point of the cheek and is thus easily palpable.

Anterosuperior to the cheek the margins of the orbit can be palpated, being particularly marked deep to the eyebrow.

The mandible

The mandible (Fig. 5.4) or lower jaw completes the skull. It consists of a horizontal convex body, with two upwardly projecting rami from the posterior ends of the body. The rami and body all present an outer and an inner surface, with the body having superior and inferior borders, while the rami have anterior and posterior borders. The outer surface of the body is slightly concave from top to bottom and markedly convex from side to side, and may show a vertical line in the midline where the two halves have fused. Towards the front of the body on each side is the mental foramen transmitting the mental nerve and vessels. The inner surface of the body is divided by a slightly raised ridge of bone, the mylohyoid line, into upper and lower areas. Behind the front of the mandible, above this line, is the sublingual fossa which lodges the sublingual salivary gland, while below this line, in the middle third of the body, is the submandibular fossa for the submandibular gland. On the inner surface of the anterior midline region are two pairs of bony projections, the genial (mental) spines. The upper pair lie above the mylohyoid line and give rise to part of the tongue (genioglossus), while the lower pair lie below the line and give attachment to geniohyoid. The superior border of the body consists of a series of sockets for the teeth. The bone behind the third molar is thickened as it joins the anterior border of the ramus. The lower border of the body is thick and rounded, but shows two fossae anteriorly, one each side of the midline. These are the digastric fossae, each giving attachment to the anterior belly of digastric. Posteriorly the body meets the lower posterior part of the ramus at the angle, which tends to be everted in males and inverted in females. Just in front of the angle the lower border is notched where the facial artery crosses it to enter the face.

Figure 5.4 (A) Lateral and (B) medial views of the mandible.

Each ramus is continuous with the body, but is flatter. The inner surface is marked by the mandibular foramen, which is partially covered by a small bony process, the lingula. The inferior alveolar nerve and vessels enter the mandible at the mandibular foramen, giving off mylohyoid branches before they do so. These pass in the downward and forward running mylohyoid groove, also seen on the inner surface of the ramus. The area behind the mylohyoid groove towards the angle of the mandible is roughened for the attachment of medial pterygoid. The sphenomandibular ligament attaches to the lingula. The outer surface of the ramus is roughened for the attachment of masseter. The posterior border is thick and rounded, particularly at its lower end, and may show a shallow fossa for part of the parotid gland. The anterior border is also thick inferiorly but becomes a thin pointed projection superiorly, the coronoid process. The concave superior border passes between the coronoid process anteriorly and the condylar process (head) posteriorly. The head, which articulates with the mandibular fossa of the temporal bone, is much broader transversely than from front to back, and is marked at its lateral and medial ends by prominent tubercles.

Palpation

The entire length and depth of the mandible are palpable with the angle posteriorly, being prominent, particularly in males, even though the posterior border and lateral surface of the ramus is mainly covered by muscle and part of the parotid gland. The lateral tubercle on the condyle can be palpated anterior to the tragus of the ear below the posterior part of the zygomatic arch. Identification is easier with the subject opening and closing the mouth, when the condyle can be felt gliding forwards on the articular surface of the temporal bone.

The hyoid bone

The U-shaped hyoid bone (Fig. 5.5) is deficient posteriorly and suspended in the neck by muscle attachments below the tongue and above the larynx (see Fig. 5.8). Being attached to the tongue, it moves up and down with the tongue during swallowing. Because the hyoid is also firmly attached to the larynx by the thyrohyoid membrane, when the hyoid moves upwards it carries the larynx with it. The hyoid consists of a body anteriorly, a pair of greater horns which project upwards and backwards from the body, and a pair of lesser horns which also project upwards and backwards from the junction of the body with the greater horns. The lesser horns and upper part of the body of the hyoid are derived from the second pharyngeal arch, while the greater horns and lower part of the body are from the third pharyngeal arch. The stylohyoid ligament, also from the second arch, attaches to the lesser horn. The hyoid is connected by muscles and ligaments to the tongue, the mandible, base of the skull (styloid process), thyroid cartilage and sternum.

Figure 5.5 Superior (left) and lateral (right) views of the hyoid bone.

Figure 5.8 Anterior view of the suprahyoid muscles.

Ossification

A pair of ossification centres, which soon unite, appear in the body shortly before birth, as well as a single centre for each greater horn. During the first year centres appear for the lesser horns. Not until middle age do the body and greater horns fuse, and only in old age do the lesser horns fuse with the remainder.

Palpation

The two greater horns can be felt through the skin below the mandible if firm pressure is applied between the finger and the thumb above the thyroid cartilage of the larynx. In this position the bone can be moved from side to side. The body can be palpated in the front of the neck about 2 cm above the laryngeal prominence with the chin elevated.

Muscles

Muscles which change the shape of the face

The muscles which change the shape of the face are commonly known as the muscles of facial expression (Fig. 5.6). Although with the contraction of individual or groups of muscles, human beings can smile, frown, look happy or sad, and generally convey many emotions, it must be remembered that the primary action of the majority of these muscles is to dilate (open) or constrict (close) the eyes, nose and mouth. In addition, one of these muscles, buccinator, plays an important role in mastication (p. 552), while others are involved in forming and shaping the sounds produced by the larynx into recognizable words.

Figure 5.6 Facial muscles.

To be able to perform these many complex actions it is not surprising that the majority of the muscles have only one attachment to bone, the other being to the superficial fascia and skin. Consequently, when the facial muscles contract to dilate or constrict the eyes, nose or mouth, their secondary action has a profound effect on facial expression, through which an individual can convey their emotions to the world at large, and recognize the feelings of others.

All of the muscles of facial expression are derived from the second pharyngeal arch, and consequently are supplied by branches of the facial (seventh cranial) nerve. Damage to branches of the facial nerve will result in a loss of tone in the muscles supplied by that branch, with a consequent sagging of that part of the face. The skin of the face is supplied by the ophthalmic, maxillary and mandibular branches of the trigeminal (fifth cranial) nerve.

These muscles are best considered in groups in relation to whether they act upon the eyes, nose or mouth. Those muscles which do not adhere to this plan are considered individually.

Movements of the eyebrows

Occipitofrontalis

Corrugator supercilii

Procerus

Occipitofrontalis

Raising the eyebrows, as in an expression of surprise, is produced by occipitofrontalis (Fig. 5.6), which is in two parts united by the strong aponeurosis of the scalp. The posterior part arises from the outer part of the superior nuchal line and the mastoid part of the temporal bone from where it passes to attach to the aponeurosis. The larger anterior part of the muscle runs from the aponeurosis to the superficial fascia of the forehead and eyebrow, interlacing with orbicularis oculi.

Action

The contraction of the posterior or anterior bellies of occipitofrontalis pulls the scalp backwards or forwards respectively. The anterior bellies acting from the aponeurosis raise the eyebrows either together in surprise, or individually as in a quizzical expression. This action produces the deep transverse lines on the forehead.

Corrugator supercilii

This small muscle (Fig. 5.6) arises from the medial end of the superciliary arch (eyebrow), blending with orbicularis oculi. Its fibres run upwards and laterally to attach to the skin of the eyebrows. On contraction they pull the two eyebrows together medially and downwards as in a frown, so producing the deep vertical wrinkles between the eyebrows.

Procerus

These small muscles, are continuous with each other across the midline. They run from the lower part of the nasal bone to the skin over the lower part of the forehead, intermingling with the anterior belly of occipitofrontalis (Fig. 5.6). On contraction they pull down the medial part of the eyebrow producing transverse wrinkles at the root of the nose.

Muscles around the eye

Orbicularis oculi

One of the most complex muscles of the face and by far the most important around the eye (Fig. 5.6). It consists of three parts – an orbital part, a palpebral part and a lacrimal part. The orbital part surrounds the orbit, spreading onto the forehead, temple and cheek, taking its origin from the medial margin of the orbit and the medial palpebral ligament. The fibres arising from above the ligament run elliptically around the orbit to attach again below the ligament.

The palpebral part of the muscle is much thinner and lies within the eyelids. These fibres arise from the medial palpebral ligament and run outwards within the eyelids to unite at the lateral palpebral raphe.

The lacrimal part of orbicularis oculi arises from fascia behind the lacrimal sac and crest of the lacrimal bone, passing laterally to attach to the tarsal plates of each eyelid.

Action

Orbicularis oculi plays an important part in protecting the eye by washing the eyeball with tears and by firmly closing to protect the eye against insult and injury. The orbital part of the muscle, which can act independently or with other parts, draws the skin of the forehead and cheek towards the medial angle of the orbit, so that the eye is screwed up tightly as when a bright light is shone into the eyes or when dust blows towards them. This action produces the characteristic ‘crow’s feet’ at the lateral corner of the eye. The palpebral part of the muscle exerts much finer control over the individual eyelids, and by reflex or voluntary contraction pulls down the upper and raises the lower eyelid to close the eye. By pulling the eyelids medially during regular blinking, tears produced by the lacrimal gland are wiped over the surface of the eyeball to keep it moist and wash particles towards the lacrimal puncta. The small movements of the eyelids are very important in non-verbal communication between humans.

The lacrimal part of the muscle dilates the lacrimal sac.

Application

When orbicularis oculi is paralysed there is an inability to close the eye. The resultant failure of this mechanism for blinking and washing the eyeball means that the eyeball tends to become red and inflamed.

Muscles around the nose

Nasalis

Levator labii superioris alaeque nasi

Depressor septi

These small muscles around the nose (Fig. 5.6) act to dilate or constrict the nasal apertures. These actions are rudimentary in humans, but are of obvious importance in some animals, for example, camels. The dilators are nasalis and levator labii superioris alaeque nasi, which also acts on the upper lip, while the constrictor is depressor septi.

Muscles around the mouth

Orbicularis oris

Buccinator

Levator labii superioris alaeque nasi

Levator labii superioris

Zygomaticus major and minor

Levator anguli oris

Depressor anguli oris and depressor labii inferioris

Risorius and mentalis

Orbicularis oris

Surrounding the mouth (Fig. 5.6) orbicularis oris is a composite sphincter muscle with deep and superficial parts. Vertically it extends from the nasal septum to midway between the chin and lower lip. The deep fibres are continuous with buccinator, while the superficial fibres are all derived from other muscles. The deep fibres attach to both the maxilla and mandible near the lateral incisor. Between the deep and superficial layers of the muscle, the intrinsic fibres of orbicularis oris run elliptically around the mouth, having no bony attachment. Some of the superficial fibres decussate like those of buccinator.

Action

Orbicularis oris produces movements of the lips as in ‘puckering’ or whistling. It has an important role in speech by changing the shape of the mouth and lips to help form recognizable sounds. It is also important in mastication as its contraction against the teeth helps to keep food between the teeth during chewing. Contractions of the various muscles which insert into orbicularis oris change its shape.

Buccinator

Buccinator is continuous with the deep part of orbicularis oris and forms the substance of the cheek (Fig. 5.6). It arises from both the maxilla and the mandible opposite the molar teeth, and from the pterygomandibular raphe, which stretches from the pterygoid hamulus to the posterior end of the mylohyoid line. The fibres run forwards to blend with those of orbicularis oris. The medial fibres decussate posterolateral to the angle of the mouth, so that the lower fibres run to the upper lip and the upper fibres to the lower lip. The interlacing of deep buccinator fibres, with some of the superficial fibres of orbicularis oris, forms an easily felt nodule at the angle of the mouth known as the modiolus.

Action

Buccinator presses the cheek against the teeth or resists outward pressure against the cheek. The former action is important in mastication as it prevents the accumulation of food in the vestibule of the mouth – a common problem when buccinator is paralysed. When blowing up a balloon, buccinator can be felt resisting the outward pressure of air against the cheek. When the lips are protruded by orbicularis oris, buccinator causes the cheek to ‘cave in’, thereby producing a sucking action. Buccinator can also be used to pull orbicularis oris posteriorly against the teeth, or to retract the angle of the mouth to expose the premolar and molar teeth.

Palpation

If a finger is placed between the cheek and teeth, buccinator can be felt contracting as the finger is pressed against the teeth.

Levator labii superioris alaeque nasi

This muscle (Fig. 5.6) arises from the maxilla and attaches to the ala of the nose and skin, and the muscle of the upper lip.

Levator labii superioris

This muscle (Fig. 5.6) arises from the maxilla above the infraorbital foramen and attaches to the upper lip towards its lateral end.

Zygomaticus major and minor

These both arise from the zygomatic bone and attach to the upper lip near its angle (Fig. 5.6).

Levator anguli oris

This arises from the maxilla below the infraorbital foramen (Fig. 5.6), inserting into skin and muscle at the angle of the mouth. A few of its fibres may pass to the lower lip.

Action

All of these muscles when contracting on both sides raise the upper lip as in smiling; contraction of one side produces a sneer. The tone of these muscles is important in maintaining the normal horizontal position of the mouth. When paralysed, there is a gradual drooping of the angle of the mouth on the affected side leading to leakage of saliva and a constant dribble from the corner of the mouth. Zygomaticus major, assisted by levator anguli oris, raises and pulls the angle of the mouth laterally, as in laughing.

The following facial muscles have their effect on the lower lip.

Depressor anguli oris and depressor labii inferioris

Depressor anguli oris arises from the front of the mandible below the mental foramen and blends with the muscles of the lower lip at the angle of the mouth (Fig. 5.6), as well as inserting into skin. Some of its fibres may pass into the upper lip. It is continuous with platysma and overlaps depressor labii inferioris which also arises from the mandible below the mental foramen. Its fibres attach to the skin and muscle of the lower lip, the fibres from each side blending together (Fig. 5.6).

Muscles moving the mandible

The movements of the mandible are complex, involving the coordinated action of the muscles attached to it. Because of the nature of the temporomandibular joint (p. 559), four basic movements of the mandible can be identified; these being:

1. protraction – pulling the mandible forwards so that its head articulates indirectly with the articular eminence of the temporal bone

2. retraction – pulling the mandible backwards so that its head moves back into the mandibular fossa

3. elevation – closing the mouth

4. depression – opening the mouth.

Combinations of these basic movements occur in chewing and grinding. Opening the mouth, particularly against resistance, as when eating sticky foods, will involve the infrahyoid group of muscles in order to stabilize the hyoid bone and provide a firm base against which mandibular movements can be made.

Muscles elevating the mandible

Masseter

Medial pterygoid

Temporalis (p. 555)

Masseter

A flat quadrilateral muscle with deep and superficial parts (Fig. 5.7A). The superficial part arises from the zygomatic process of the maxilla and the anterior two-thirds of the zygomatic arch, the muscle fibres running downwards and backwards to attach to the outer surface of the angle of the mandible, extending onto the lower half of the outer surface of the ramus. The deeper part arises from the deep surface of the zygomatic arch, the fibres of which run downwards and backwards to attach to the ramus and coronoid process of the mandible.

Digastric

Digastric as its name suggests, has two bellies, anterior and posterior, united by an intermediate tendon (Fig. 5.8). The posterior belly passes downwards and forwards from the medial surface of the mastoid process in close association with stylohyoid, to an intermediate tendon which passes through the insertion of stylohyoid and is held by an aponeurotic sling to the upper surface of the hyoid. The anterior belly of the muscle runs upwards and forwards to attach to the digastric fossa on the lower border of the mandible close to the symphysis menti.

Nerve supply

The posterior belly is supplied by the facial (seventh cranial) nerve, and the anterior belly by the nerve to mylohyoid from the inferior alveolar branch of the mandibular division of the trigeminal (fifth cranial) nerve.

Action

If the hyoid is fixed, digastric can depress the mandible thereby opening the mouth, an action required when acting against resistance. With the mandible elevated, digastric also helps in retraction of the mandible. With the mandible fixed, digastric raises the hyoid and with it the larynx, an important action in swallowing.

Mylohyoid

The mylohyoid (Fig. 5.8) arises from the mylohyoid line on the inner surface of the body of the mandible. Its fibres run downwards and medially towards the midline to insert into a median fibrous raphe which extends from the symphysis menti to the upper surface of the body of the hyoid bone. The two mylohyoid muscles thus give rise to a muscular sheet which forms the floor of the mouth.