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GENERAL CONSIDERATIONS ON MUSCLES
&
SKULL
28. September.2011 Wednesday
GENERAL CONSIDERATIONS ON MUSCLES
The muscular system consists of all the muscles of the body. The disciplined related to the study of
muscles is myology. Musculus (muscle) is derived from the word mus-mouse; musculus- little mouse. All
skeletal muscles are composed of one specific type of muscle tissue. These muscles move the skeleton,
therefore, move the body parts. Other types of muscle tissue constitute a few named muscles (e.g., the
ciliary and detrusor muscles, and the arrector muscles of hairs) and form important components of the organs
of other systems, including the cardiovascular, alimentary, genitourinary, integumentary, and visual systems.
Types of Muscle (Muscle Tissue)
Three types of muscle are described based on distinct characteristics relating to:
 Whether it is normally willfully controlled (voluntary vs. involuntary).
 Whether it appears striped or unstriped when viewed under a microscope (striated vs. smooth or
unstriated).
 Whether it is located in the body wall (soma) and limbs or makes up the hollow organs (viscera) of
the body cavities or blood vessels (somatic vs. visceral).
There are three muscle types:
 Skeletal striated muscle is voluntary somatic muscle that makes up the gross skeletal muscles that
compose the muscular system, moving or stabilizing bones and other structures (e.g., the eyeballs).
Striated muscles are innervated by the somatic nervous system.
 Cardiac striated muscle is involuntary visceral muscle that forms most of the walls of the heart and
adjacent parts of the great vessels, such as the aorta, and pumps blood.
 Smooth muscle (unstriated muscle) is involuntary visceral muscle that forms part of the walls of
most vessels and hollow organs (viscera), moving substances through them by coordinated sequential
contractions (pulsations or peristaltic contractions). Non-striated and cardiac muscle are innervated
by the autonomic nervous system.
Skeletal Muscles
Form, Features, and the Naming of muscles
All skeletal muscles, commonly referred to simply as “muscles,” have fleshy, reddish, contractile
portions (one or more heads or bellies) composed of skeletal striated muscle. Some muscles are fleshy
throughout, but most also have white non-contractile portions (tendons), composed mainly of organized
collagen bundles, that provide a means of attachment. When referring to the length of a muscle, both the
belly and the tendons are included. In other words, a muscle's length is the distance between its attachments.
Most skeletal muscles are attached directly or indirectly to bones, cartilages, ligaments, or fascias or
to some combination of these structures. Some muscles are attached to organs (the eyeball, for example),
skin (such as facial muscles), and mucous membranes (intrinsic tongue muscles). Muscles are organs of
locomotion (movement), but they also provide static support, give form to the body, and provide heat.
The architecture and shape of muscles vary. The tendons of some muscles form flat sheets, or
aponeuroses, that anchor the muscle to the skeleton (usually a ridge or a series of spinous processes) and/or
to deep fascia (such as the latissimus dorsi muscle of the back), or to the aponeurosis of another muscle
(such as the oblique muscles of the anterolateral abdominal wall).
Muscle terminology
Many terms provide information about a structure's shape, size, location, or function or about the
resemblance of one structure to another.
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 Most muscles are named on the basis of their function or the bones to which they are attached. The
abductor digiti minimi muscle, for example, abducts the little finger. The sternocleidomastoid muscle
(G. kleidos, bolt or bar, clavicle) attaches inferiorly to the sternum and clavicle and superiorly to the
mastoid process of the temporal bone of the cranium. Another example is the levator scapulae which
elevates the scapula (L. shoulder blade).
 Some muscles have descriptive names to indicate their main characteristics. The deltoid muscle,
which covers the point of the shoulder, is triangular, like the symbol for delta, the fourth letter of the
Greek alphabet. The suffix -oid means “like”; therefore, deltoid means like delta.
 Other muscles are named on the basis of their position (medial, lateral, anterior, posterior) or length
(brevis, short; longus, long).
 Some muscles are named according to their shape—the piriformis muscle, for example, is pear shaped
(L. pirum, pear + L. forma, shape or form).
 Other muscles are named according to their location. The temporal muscle is in the temporal region
(temple) of the cranium (skull).
Muscles may be described or classified according to their shape, for which a muscle may also be named:
 Flat muscles have parallel fibers often with an aponeurosis—for example, the external oblique (broad
flat muscle). The sartorius is a narrow flat muscle with parallel fibers.
 Pennate muscles are feather-like (L. pennatus, feather) in the arrangement of their fascicles, and may be
unipennate, bipennate, or multi-pennate—for example, the extensor digitorum longus (unipennate), the
rectus femoris (bipennate), and deltoid (multi-pennate).
 Fusiform muscles are spindle shaped with a round, thick belly (or bellies) and tapered ends—for
example, biceps brachii.
 Convergent muscles arise from a broad area and converge to form a single tendon—for example, the
pectoralis major.
 Quadrate muscles have four equal sides (L. quadratus, square)—for example, the rectus abdominis,
between its tendinous intersections.
 Circular or sphincteral muscles surround a body opening or orifice, constricting it when contracted—
for example, orbicularis oculi (closes the eyelids).
 Multi-headed or multi-bellied muscles have more than one head of attachment or more than one
contractile belly, respectively. Biceps muscles have two heads of attachment (e.g., the biceps brachii),
triceps muscles have three heads (e.g., triceps brachii), and the digastric and gastrocnemius muscles have
two bellies.
Contraction of muscles
Skeletal muscles function by contracting; they pull and never push. When a muscle contracts and
shortens, one of its attachments usually remains fixed while the other (more mobile) attachment is pulled
toward it, often resulting in movement. Attachments of muscles are commonly described as the origin and
insertion; the origin is usually the proximal end of the muscle, which remains fixed during muscular
contraction, and the insertion is usually the distal end of the muscle, which is movable. However, this is not
always the case. Some muscles can act in both directions under different circumstances. For example, when
doing pushups, the distal end of the upper limb (the hand) is fixed (on the floor) and the proximal end of the
limb and the trunk are being moved.
Reflexive Contraction: Although skeletal muscles are also referred to as voluntary muscles, certain aspects
of their activity are automatic (reflexive) and therefore not voluntarily controlled. Examples are the
respiratory movements of the diaphragm, controlled most of the time by reflexes stimulated by the levels of
oxygen and carbon dioxide in the blood (although we can willfully control it within limits), and the myotatic
reflex, which results in movement after a muscle stretch produced by tapping a tendon with a reflex hammer.
Tonic Contraction: Even when “relaxed” the muscles of a conscious individual are almost always
slightly contracted. This slight contraction, called muscle tone (tonus), does not produce movement or active
resistance (as phasic contraction does) but gives the muscle a certain firmness, assisting the stability of joints
and the maintenance of posture, while keeping the muscle ready to respond to appropriate stimuli. Muscle
tone is usually absent only when unconscious (as during deep sleep or under general anesthesia) or after a
nerve lesion resulting in paralysis.
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Phasic Contraction: There are two main types of phasic (active) muscle contractions: (1) isotonic
contractions, in which the muscle changes length in relationship to the production of movement, and (2)
isometric contractions, in which muscle length remains the same—no movement occurs, but the force
(muscle tension) is increased above tonic levels to resist gravity or other antagonistic force. When a muscle
contracts its length decreases by 1/3 or ½.
Whereas the structural unit of a muscle is a skeletal striated muscle fiber, the functional unit of a
muscle is a motor unit, consisting of a motor neuron and the muscle fibers it controls. When a motor neuron
in the spinal cord is stimulated, it initiates an impulse that causes all the muscle fibers supplied by that motor
unit to contract simultaneously. The number of muscle fibers in a motor unit varies from one to several
hundred. The number of fibers varies according to the size and function of the muscle. Large motor units, in
which one neuron supplies several hundred muscle fibers, are in the large trunk and thigh muscles. In the
smaller eye and hand muscles, where precision movements are required, the motor units include only a few
muscle fibers. Movement (phasic contraction) results from the activation of an increasing number of motor
units, above the level required to maintain muscle tone.
Functions of muscles
Muscles serve specific functions in moving and positioning the body.

A prime mover (agonist) is the main muscle responsible for producing a specific movement of the
body. It contracts concentrically to produce the desired movement, doing most of the work (expending most
of the energy) required. In most movements, there is a single prime mover, but some movements involve
two prime movers working in equal measure.

A fixator steadies the proximal parts of a limb through isometric contraction while movements are
occurring in distal parts.

A synergist complements the action of a prime mover. It may directly assist a prime mover, providing a
weaker or less mechanically advantaged component of the same movement, or it may assist indirectly, by
serving as a fixator of an intervening joint when a prime mover passes over more than one joint, for
example. It is not unusual to have several synergists assisting a prime mover in a particular movement.

An antagonist is a muscle that opposes the action of another muscle. A primary antagonist directly
opposes the prime mover, but synergists may also be opposed by secondary antagonists. As the active
movers concentrically contract to produce a movement, antagonists eccentrically contract, relaxing
progressively in coordination to produce a smooth movement.
The same muscle may act as a prime mover, antagonist, synergist, or fixator under different conditions.
Nerves and arteries to muscles
Variation in the nerve supply of muscles is rare; it is a nearly constant relationship. In the limb, muscles of
similar actions are generally contained within a common fascial compartment and share innervation by the
same nerves. Nerves supplying skeletal muscles (motor nerves) usually enter the fleshy portion of the
muscle (vs. the tendon), almost always from the deep aspect (so the nerve is protected by the muscle it
supplies). The blood supply of muscles is not as constant as the nerve supply and is usually multiple.
Arteries generally supply the structures they contact.
Cardiac Striated Muscle
Cardiac striated muscle forms the muscular wall of the heart, the myocardium. Some cardiac muscle is also
present in the walls of the aorta, pulmonary vein, and superior vena cava. Cardiac striated muscle
contractions are not under voluntary control. Heart rate is regulated intrinsically by a pacemaker, an impulseconducting system composed of specialized cardiac muscle fibers; they, in turn, are influenced by the
autonomic nervous system (ANS).
Smooth Muscle
Smooth muscle, named for the absence of striations in the appearance of the muscle fibers under
microscopy, forms a large part of the middle coat or layer (tunica media) of the walls of blood vessels
(above the capillary level). Consequently, it occurs in all vascularized tissue. It also makes up the muscular
parts of the walls of the alimentary tract and ducts. Smooth muscle is found in skin, forming the arrector
muscles of hairs associated with hair follicles, and in the eyeball, where it controls lens thickness and pupil
size.
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Fascias (L. fasciae) constitute the wrapping, packing, and insulating materials of the deep structures of the
body. Underlying the subcutaneous tissue (superficial fascia) almost everywhere is the deep fascia. The deep
fascia is a dense, organized connective tissue layer, devoid of fat, that covers most of the body parallel to
(deep to) the skin and subcutaneous tissue. In the limbs, groups of muscles with similar functions sharing the
same nerve supply are located in fascial compartments, separated by thick sheets of deep fascia, called
intermuscular septa, that extend centrally from the surrounding fascial sleeve to attach to bones. These
compartments may contain or direct the spread of an infection or a tumor.
Subserous fascia, with varying amounts of fatty tissue, lies between the internal surfaces of the
musculoskeletal walls and the serous membranes lining the body cavities. These are the endothoracic,
endoabdominal, and endopelvic fascias; the latter two may be referred to collectively as extraperitoneal
fascia.
Clinical Notes
Muscle Tone
Determination of the tone of a muscle is an important clinical examination. If a muscle is flaccid, then either
the afferent, the efferent, or both neurons involved in the reflex arc necessary for the production of muscle
tone have been interrupted. If, conversely, the muscle is found to be hypertonic, the possibility exists of a
lesion involving higher motor neurons in the spinal cord or brain.
Muscle Shape and Form
The general shape and form of muscles should also be noted, since a paralyzed muscle or one that is not used
(such as occurs when a limb is immobilized in a cast) quickly atrophies and changes shape. In the case of the
limbs, it is always worth remembering that a muscle on the opposite side of the body can be used for
comparison.
Electromyography (EMG)
Electromyography (EMG) is a technique for evaluating and recording the electrical activity produced by
skeletal muscles. It is a diagnostic procedure to assess the health of muscles and the nerve cells that control
them (motor neurons). An EMG translates these signals into graphs, sounds or numerical values that a
specialist interprets. EMG results can reveal nerve dysfunction, muscle dysfunction or problems with nerveto-muscle signal transmission. EMG is performed using an instrument called an electromyograph, to
produce a record called an electromyogram.
SKULL BONES
The cranium (skull) is the skeleton of the head. A series of bones form its two parts, the
neurocranium and viscerocranium (facial bones).
The neurocranium is the bony case of the brain and its membranous coverings, the cranial
meninges. It also contains proximal parts of the cranial nerves and the vasculature of the brain. The
neurocranium in adults is formed by a series of eight bones: four singular bones centered on the midline
(frontal, ethmoidal, sphenoidal, and occipital) and two sets of bones occurring as bilateral pairs (temporal
and parietal).
Occipital.
Two Parietals.
Frontal.
Cranium, 8 bones
Two Temporals.
Sphenoidal.
Ethmoidal.
Skull, 22 bones
Face, 14 bones
Two Nasals.
Two Maxillæ.
Two Lacrimals.
Two Zygomatics.
Two Palatines.
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Two Inferior Nasal Conchæ.
Vomer.
Mandible.
The neurocranium has a dome-like roof, the calvaria (skullcap), and a floor or cranial base
(basicranium). The bones making the calvaria are primarily flat bones (frontal, parietal, and occipital). The
bones contributing to the cranial base are primarily irregular bones with substantial flat portions (sphenoidal
and temporal). The ethmoid bone is an irregular bone that makes a relatively minor midline contribution to
the neurocranium but is primarily part of the viscerocranium.
The viscerocranium (facial skeleton) comprises the facial bones. The viscerocranium forms the
anterior part of the cranium and consists of the bones surrounding the mouth (upper and lower jaws),
nose/nasal cavity, and most of the orbits (eye sockets or orbital cavities). The viscerocranium consists of 15
irregular bones: 3 singular bones centered on or lying in the midline (mandible, ethmoid, and vomer) and 6
bones occurring as bilateral pairs (maxillae; inferior nasal conchae; and zygomatic, palatine, nasal, and
lacrimal bones).
The maxillae and mandible house the teeth—that is, they provide the sockets and supporting bone for the
maxillary and mandibular teeth. The maxillae contribute the greatest part of the upper facial skeleton,
forming the skeleton of the upper jaw, which is fixed to the cranial base. The mandible forms the skeleton of
the lower jaw, which is movable because it articulates with the cranial base at the temporomandibular joints.
Several bones of the cranium (frontal, temporal, sphenoid, and ethmoid bones) are pneumatized
bones, which contain air spaces (air cells or large sinuses), presumably to decrease their weight. The total
volume of the air spaces in these bones increases with age.
Ossa Cranii
The Frontal Bone
The frontal bone resembles a cockle-shell in form. It consists of two portions—a vertical portion,
the squama, corresponding with the region of the forehead; and an orbital or horizontal portion, which
enters into the formation of the roofs of the orbital and nasal cavities. The frontal bone, specifically its
squamous (flat) part, forms the skeleton of the forehead, articulating inferiorly with the nasal and zygomatic
bones. The frontal bone also articulates with the lacrimal, ethmoid, and sphenoids; a horizontal portion of
bone (orbital part) forms both the roof of the orbit and part of the floor of the anterior part of the cranial
cavity.
The intersection of the frontal and the nasal bones is the nasion (L. nasus, nose), which in most
people is related to a distinctly depressed area (bridge of nose). The nasion is one of many craniometric
points that are used radiographically in medicine (or on dry crania in physical anthropology) to make cranial
measurements, compare and describe the topography of the cranium, and document abnormal variations.
The supra-orbital margin of the frontal bone, the angular boundary between the squamous and the
orbital parts, has a supra-orbital foramen or notch in some crania for passage of the supra-orbital nerve
and vessels. Just superior to the supra-orbital margin is a ridge, the superciliary arch that extends laterally on
each side from the glabella.
The internal surface of the squama frontalis of the frontal bone is concave and presents in the upper
part of the middle line a vertical groove, the sagittal sulcus, the edges of which unite below to form a ridge,
the frontal crest. The frontal crest is a median bony extension of the frontal bone. At its base is the foramen
cecum of the frontal bone, which gives passage to vessels during fetal development but is insignificant
postnatally. Nasal process is the downward projection of the nasal part of the frontal bone which terminates
as the nasal spine.
The Parietal Bones
The parietal bones form, by their union, the sides and roof of the cranium. Each bone is irregularly
quadrilateral in form. The external surface is convex, smooth, and marked near the center by an eminence,
the parietal eminence (tuber parietale). Crossing the middle of the bone in an arched direction are two
curved lines, the superior and inferior temporal lines. The parietal foramen is a small, inconstant
aperture located posteriorly in the parietal bone near the sagittal suture.
The Temporal Bones
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The temporal bones are situated at the sides and base of the skull. Each consists of five parts, viz.,
the squama, the petrous, mastoid, and tympanic parts, and the styloid process.
The temporal fossa is bounded superiorly and posteriorly by the superior and inferior temporal lines,
anteriorly by the frontal and zygomatic bones, and inferiorly by the zygomatic arch. The superior border of
this arch corresponds to the inferior limit of the cerebral hemisphere of the brain. The zygomatic arch is
formed by the union of the temporal process of the zygomatic bone and the zygomatic process of the
temporal bone. In the anterior part of the temporal fossa, superior to the midpoint of the zygomatic arch, is
a clinically important area of bone junctions: the pterion (G. pteron, wing). It is usually indicated by an Hshaped formation of sutures that unite the frontal, parietal, sphenoid (greater wing), and temporal bones.
The external acoustic opening (pore) is the entrance to the external acoustic meatus (canal),
which leads to the tympanic membrane (eardrum). The mastoid process of the temporal bone is
posteroinferior to the external acoustic opening. The mastoid processes provide for muscle attachments
Anteromedial to the mastoid process is the styloid process of the temporal bone, a slender needle-like,
pointed projection. The infratemporal fossa is an irregular space inferior and deep to the zygomatic arch
and the mandible and posterior to the maxilla.
The large opening between the occipital bone and the petrous part of the temporal bone is the jugular
foramen, from which the internal jugular vein (IJV) and several cranial nerves (CN IX-CN XI) emerge from
the cranium. At the base of the petrous ridge of the temporal bone; the jugular foramen transmits several
cranial nerves in addition to the sigmoid sinus that exits the cranium as the internal jugular vein (IJV).
Anterosuperior to the jugular foramen is the internal acoustic meatus for the facial and vestibulocochlear
nerves (CN VIII) and the labyrinthine artery. The entrance to the carotid canal for the internal carotid
artery is just anterior to the jugular foramen.. The stylomastoid foramen, transmitting the facial nerve (CN
VII) and stylomastoid artery, lies posterior to the base of the styloid process.
The Sphenoid Bone
The sphenoid bone is situated at the base of the skull in front of the temporals and basilar part of the
occipital. It somewhat resembles a bat with its wings extended, and is divided into a median portion or
body, two great and two small wings extending outward from the sides of the body, and two pterygoid
processes which project from it below.
The greater and lesser wings of the sphenoid spread laterally from the lateral aspects of the body of
the bone. The pterygoid processes, consisting of lateral and medial pterygoid plates, extend inferiorly on
each side of the sphenoid from the junction of the body and greater wings.
The sphenoidal crests are formed mostly by the sharp posterior borders of the lesser wings of the
sphenoid bones. The sphenoidal crests end medially in two sharp bony projections, the anterior clinoid
processes. A variably prominent ridge, the limbus of the sphenoid forms the anterior boundary of the
transversely oriented prechiasmatic sulcus extending between the right and the left optic canals.
The sella turcica (L. Turkish saddle) is the saddle-like bony formation on the upper surface of the
body of the sphenoid, which is surrounded by the anterior and posterior clinoid processes. Clinoid means
“bedpost,” and the four processes (two anterior and two posterior) surround the hypophysial fossa, the “bed”
of the pituitary gland, like the posts of a four-poster bed. The sella turcica is composed of three parts:
 The tuberculum sellae (horn of saddle): a variable slight to prominent median elevation forming the
posterior boundary of the prechiasmatic sulcus and the anterior boundary of the hypophysial fossa. It lies
behind the chiasmatic groove. The hypophysial fossa (pituitary fossa): a median depression (seat of saddle)
in the body of the sphenoid that accommodates the pituitary gland (L. hypophysis).
 The dorsum sellae (back of saddle): a square plate of bone projecting superiorly from the body of the
sphenoid. It forms the posterior boundary of the sella turcica, and its prominent superolateral angles make up
the posterior clinoid processes.
On each side of the body of the sphenoid, a crescent of four foramina perforate the roots of the cerebral
surfaces of the greater wings of the sphenoids:
 Superior orbital fissure: Located between the greater and the lesser wings, it opens anteriorly into the
orbit.
 Foramen rotundum (round foramen): Located posterior to the medial end of the superior orbital
fissure.
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 Foramen ovale (oval foramen): A large foramen posterolateral to the foramen rotundum, it opens
inferiorly into the infratemporal fossa.
 Foramen spinosum (spinous foramen): Located posterolateral to the foramen ovale, it also opens into
the infratemporal fossa in relationship to the spine of the sphenoid.
The foramen lacerum (lacerated or torn foramen) is not part of the crescent of foramina. This ragged
foramen lies posterolateral to the hypophysial fossa and is an artifact of a dried cranium. In life, it is closed
by a cartilage plate. Extending posteriorly and laterally from the foramen lacerum is a narrow groove for the
greater petrosal nerve on the anterosuperior surface of the petrous part of the temporal bone. There is also a
small groove for the lesser petrosal nerve.
The Occipital bone
The occipital bone is situated at the back and lower part of the cranium, is trapezoid in shape and
curved on itself. It is pierced by a large oval aperture, the foramen magnum, through which the cranial cavity
communicates with the vertebral canal. The curved, expanded plate behind the foramen magnum is named
the squama; the thick, somewhat quadrilateral piece in front of the foramen is called the basilar part, whilst
on either side of the foramen is the lateral portion. The cranial base is formed posteriorly by the occipital
bone, which articulates with the sphenoid bone anteriorly.
The external occipital protuberance, is usually easily palpable in the median plane; however,
occasionally (especially in females) it may be inconspicuous. A craniometric point defined by the tip of the
external protuberance is the inion (G. nape of neck). The external occipital crest descends from the
protuberance toward the foramen magnum, the large opening in the basal part of the occipital bone The four
parts of the occipital bone are arranged around the foramen magnum, the most conspicuous feature of the
cranial base. The major structures passing through this large foramen are the spinal cord (where it becomes
continuous with the medulla oblongata of the brain), the meninges (coverings) of the brain and spinal cord,
the vertebral arteries, the anterior and posterior spinal arteries, and the spinal accessory nerve (CN XI). The
superior nuchal line, marking the superior limit of the neck, extends laterally from each side of the
protuberance; the inferior nuchal line is less distinct. In the center of the occiput, lambda indicates the
junction of the sagittal and the lambdoid sutures.
Cruciate eminence: divides the interior surface of the occipital bone into four fossae. On the lateral
parts of the occipital bone are two large protuberances, the occipital condyles, by which the cranium
articulates with the vertebral column.
From the dorsum sellae there is a marked incline, the clivus (Latin for "slope"), in the center of the
anterior part of the fossa leading to the foramen magnum. The clivus is a shallow depression behind the
dorsum sellæ that slopes obliquely backward. Posterior to foramen magnum, the posterior cranial fossa is
partly divided by the internal occipital crest into bilateral large concave impressions, the cerebellar fossae.
The internal occipital crest ends in the internal occipital protuberance. The hypoglossal canal for the
hypoglossal nerve (CN XII) is superior to the anterolateral margin of the foramen magnum.
The Ethmoid Bone
The etmoid bone is exceedingly light and spongy, and cubical in shape; it is situated at the anterior
part of the base of the cranium, between the two orbits, at the roof of the nose, and contributes to each of
these cavities. It consists of four parts: a horizontal or cribriform plate, forming part of the base of the
cranium; a perpendicular plate, constituting part of the nasal septum; and two lateral masses or
labyrinths. On each side of the ridge called crista galli, located in the frontal bone, is the sieve-like
cribriform plate of the ethmoid. Its numerous tiny foramina transmit the olfactory nerves (CN I) from the
olfactory areas of the nasal cavities to the olfactory bulbs of the brain, which lie on this plate. The crista
galli (L. crest of the cock) is a thick, median ridge of bone posterior to the foramen cecum (frontal bone),
which projects superiorly from the ethmoid.
Cranial Fossas
Anterior cranial fossa
The inferior and anterior parts of the frontal lobes of the brain occupy the anterior cranial fossa, the
shallowest of the three cranial fossae. The fossa is formed by the frontal bone anteriorly, the ethmoid bone in
the middle, and the body and lesser wings of the sphenoid posteriorly. The greater part of the fossa is formed
by the orbital parts of the frontal bone, which support the frontal lobes of the brain and form the roofs of the
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orbits. This surface shows sinuous impressions (brain markings) of the orbital gyri (ridges) of the frontal
lobes.
Middle cranial fossa
The butterfly-shaped middle cranial fossa has a central part composed of the sella turcica on the body
of the sphenoid and large, depressed lateral parts on each side. The middle cranial fossa is posteroinferior to
the anterior cranial fossa, separated from it by the sharp sphenoidal crests laterally and the sphenoidal limbus
centrally. The bones forming the lateral parts of the fossa are the greater wings of the sphenoid and
squamous parts of the temporal bones laterally and the petrous parts of the temporal bones posteriorly. The
lateral parts of the middle cranial fossa support the temporal lobes of the brain. The boundary between the
middle and the posterior cranial fossae is the superior border of the petrous part of the temporal bone
laterally and a flat plate of bone, the dorsum sellae of the sphenoid, medially.
Posterior cranial fossa
The posterior cranial fossa, the largest and deepest of the three cranial fossae, lodges the cerebellum,
pons, and medulla oblongata. The posterior cranial fossa is formed mostly by the occipital bone, but the
dorsum sellae of the sphenoid marks its anterior boundary centrally and the petrous and mastoid parts of the
temporal bones contribute its anterolateral “walls.” Posterior to foramen magnum, the posterior cranial fossa
is partly divided by the internal occipital crest into bilateral large concave impressions, the cerebellar fossae.
The Facial Bones
The Nasal Bones: are two small oblong bones, varying in size and form in different individuals; they are
placed side by side at the middle and upper part of the face, and form, by their junction, “the bridge” of the
nose. Each has two surfaces and four borders.
The Maxillæ (Upper Jaw): are the largest bones of the face, excepting the mandible, and form, by their
union, the whole of the upper jaw. Each assists in forming the boundaries of three cavities, viz., the roof of
the mouth, the floor and lateral wall of the nose and the floor of the orbit; it also enters into the formation of
two fossæ, the infratemporal and pterygopalatine, and two fissures, the inferior orbital and pterygomaxillary.
Each bone consists of a body and four processes—zygomatic, frontal, alveolar, and palatine. The maxillae
form the upper jaw; their alveolar processes include the tooth sockets (alveoli) and constitute the supporting
bone for the maxillary teeth. The two maxillae are united at the intermaxillary suture in the median plane.
The maxillae surround most of the piriform aperture and form the infra-orbital margins medially. They have
a broad connection with the zygomatic bones laterally and an infraorbital foramen inferior to each orbit for
passage of the infra-orbital nerve and vessels.
The Lacrimal Bone: the smallest and most fragile bone of the face is situated at the front part of the medial
wall of the orbit.
The Zygomatic Bone (Malar Bone): is small and quadrangular, and is situated at the upper and lateral part
of the face: it forms the prominence of the cheek, part of the lateral wall and floor of the orbit, and parts of
the temporal and infratemporal fossæ.
The zygomatic bones (cheek bones, malar bones), forming the prominences of the cheeks, lie on the
inferolateral sides of the orbits and rest on the maxillae. The anterolateral rims, walls, floor, and much of the
infra-orbital margins of the orbits are formed by these quadrilateral bones. A small zygomaticofacial
foramen pierces the lateral aspect of each bone. The zygomatic bones articulate with the frontal, sphenoid,
and temporal bones and the maxillae.
Inferior to the nasal bones is the pear-shaped piriform aperture, the anterior nasal opening in the cranium.
The bony nasal septum can be observed through this aperture, dividing the nasal cavity into right and left
parts. On the lateral wall of each nasal cavity are curved bony plates, the nasal conchae.
The Palatine Bone: is situated at the back part of the nasal cavity between the maxilla and the pterygoid
process of the sphenoid. It contributes to the walls of three cavities: the floor and lateral wall of the nasal
cavity, the roof of the mouth, and the floor of the orbit.
The hard palate (bony palate) is formed by the palatal processes of the maxillae anteriorly and the
horizontal plates of the palatine bones posteriorly. The free posterior border of the hard palate projects
posteriorly in the median plane as the posterior nasal spine. Posterior to the central incisor teeth is the
incisive fossa, a depression in the midline of the bony palate into which the incisive canals open.
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The right and left nasopalatine nerves pass from the nose through a variable number of incisive canals and
foramina (they may be bilateral or merged into a single formation). Posterolaterally are the greater and
lesser palatine foramina. Superior to the posterior edge of the palate are two large openings: the choanae
(posterior nasal apertures), which are separated from each other by the vomer (L. plowshare), a flat
unpaired bone of trapezoidal shape that forms a major part of the bony nasal septum.
The Inferior Nasal Concha (Concha Nasalis Inferior; Inferior Turbinated Bone): extends horizontally
along the lateral wall of the nasal cavity.
The Mandible (Lower Jaw): the largest and strongest bone of the face, serves for the reception of the lower
teeth. It is a U-shaped bone with an alveolar process that supports the mandibular teeth. It consists of a head,
a curved, horizontal portion, the body, and two perpendicular portions, the rami, which unite with the ends
of the body nearly at right angles. Inferior to the second premolar teeth are the mental foramina for the
mental nerves and vessels. The mental protuberance, forming the prominence of the chin, is a triangular
bony elevation inferior to the mandibular symphysis (L. symphysis menti), the osseous union where the
halves of the infantile mandible fuse. Medial to the condylar process is the pterygoid fossa. Mandibular
foramen lies inferior to this fossa. Mental foramina are on the lateral sides of the mental protuberance.
The Hyoid Bone: is shaped like a horseshoe, and is suspended from the tips of the styloid processes of the
temporal bones by the stylohyoid ligaments. It has a body, greater cornu and a lesser cornu.
Vertex, the most superior point of the calvaria, is near the midpoint of the sagittal suture.
Sutura
Sutura is that form of articulation where the contiguous margins of the bones are united by a thin layer of
fibrous tissue; it is met with only in the skull
Coronal suture: Parietal bones articulate with the frontal bone in their front, forming the coronal suture.
Lambdoidal suture: Parietal bones articulate with the occipital in their behind, forming the lambdoidal
suture.
Sagittal suture: Parietal bone articulates with its the opposite side, forming the sagittal suture.
The coronal suture separates the frontal and parietal bones, the sagittal suture separates the parietal bones,
and the lambdoid suture separates the parietal and temporal bones from the occipital bone.
Important Landmarks
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Bregma: The midline point where the coronal and sagittal sutures intersect
Lambda: The midline point where the sagittal and lambdoidal sutures intersect.
Glabella: The most forward projecting point in the midline of the forehead at the level of the supraorbital ridges and above the nasofrontal suture
Pterion: The point of intersection between the frontal, sphenoid, parietal and the temporal bones
Nasion: The point of intersection between the frontonasal suture and the midsagittal plane.
Gnathion: The most anterior and lowest median point on the border of the mandible.
Fontanelles
The bones of the calvaria of a newborn infant are separated by membranous intervals. They include the
anterior and posterior fontanelles and the paired sphenoidal and mastoid fontanelles. Palpation of the
fontanelles during infancy, especially the anterior and posterior ones, enables physicians to determine the:
 Progress of growth of the frontal and parietal bones.
 Degree of hydration of an infant (a depressed fontanelle indicates dehydration).
 Level of intracranial pressure (a bulging fontanelle indicates increased pressure on the brain).
The anterior fontanelle, the largest one, is diamond or star shaped; it is bounded by the halves of the
frontal bone anteriorly and the parietal bones posteriorly. Thus it is located at the junction of the sagittal,
coronal, and frontal sutures, the future site of bregma. By 18 months of age, the surrounding bones have
fused and the anterior fontanelle is no longer clinically palpable.
The posterior fontanelle is triangular and bounded by the parietal bones anteriorly and the occipital
bone posteriorly. It is located at the junction of the lambdoid and sagittal sutures, the future site of lambda.
The posterior fontanelle begins to close during the first few months after birth; and by the end of the 1st
year, it is small and no longer clinically palpable.
The sphenoidal and mastoid fontanelles fuse during infancy and are less important clinically than the
midline fontanelles.
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The resilience of the cranial bones of infants allows them to resist forces that would produce fractures in
adults. The fibrous sutures of the calvaria also permit the cranium to enlarge during infancy and childhood.
The increase in the size of the calvaria is greatest during the first 2 years, the period of most rapid brain
development. The calvaria normally increases in capacity for 15-16 years. After this, the calvaria usually
increases slightly in size for 3-4 years as a result of bone thickening.
Clinical Notes
Head Injuries
Head injuries are a major cause of death and disability. The complications of head injuries include
hemorrhage, infection, and injury to the brain and cranial nerves. Disturbance in the level of consciousness is
the most common symptom of head injury. Head injuries occur mostly in young persons between the ages of
15 and 24 years. The major cause of brain injury varies but motor vehicle and motorcycle accidents are
prominent.
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