The twelfth rib is an atypical rib. It is the shortest rib, and one of two floating ribs.
The 12th rib has a single facet on its head for articulation with the T12 vertebra. It has a short neck and no tubercle. It also lacks a costal groove and angle. internal surface ...
The tympanic annulus is the thickened edge of the pars tensa of the tympanic membrane, anchoring it in the tympanic sulcus 3.
The tympanic annulus is formed by a fibrocartilaginous thickening of the edge of the pars tensa and has a horseshoe-shaped configuration. It is deficient ...
The tympanic membrane is a thin membrane that separates the external ear from the middle ear. It acts to transmit sound waves from air in the external auditory canal (EAC) to the ossicles of the middle ear.
The tympanic membrane is shaped like a flat cone pointing into the middle...
The tympanic part of the temporal bone is situated inferiorly to the squamous part and anteriorly to the mastoid part.
The tympanic part surrounds the external auditory meatus, forming the anterior wall, floor and some of the posterior wall of the bony external acoustic meatus. The lateral bord...
Of the seven cervical vertebrae, C3 through C6 have typical anatomy, while C7 looks very similar. C1 (atlas) and C2 (axis) have very distinct anatomical features. For a basic anatomic description of the structure a generic vertebra, see vertebrae.
small, oval-sized vertebral bodi...
Typical ribs are those numbered 2 to 10 with ribs 1, 11 and 12 considered atypical.
A typical rib is long and flat. They contain a:
Ribs have a rounded, smooth superior border. The inferior border is thin and sharp.
Given the twelve thoracic vertebrae are largely similar, most are considered typical thoracic vertebrae with the exceptions T1 and T9 to T12. For a basic anatomic description of the structure of typical vertebrae, see vertebrae.
Relative to cervical and lumbar vertebrae, thoracic...
The ulna (plural: ulnae) is one of the two long bones of the forearm. It is located medially in the supinated anatomic position. It has a larger proximal end and tapers to a smaller distal end (opposite to the radius).
Prominent features of the ulna include:
The ulnar artery is a terminal branch of the brachial artery, arising at the proximal aspect of the forearm. Along with the radial artery, it is one of the main arteries of the forearm.
origin: terminal branch of the brachial artery
location: inferior aspect of the cubital fossa
The ulnar nerve is one of the terminal branches of the brachial plexus and has motor and sensory supply to the forearm and hand.
The ulnar nerve originates as a terminal branch of the medial cord of the brachial plexus with nerve root fibres from C8-T1.
The ulnar vein is one of the two major deep veins of the forearm, along with the radial vein. As is usual in the upper and lower limbs, there are often two veins (venae comitantes) that run on either side of the ulnar artery and anastomose freely with each other.
It forms in the hand from the d...
The ulnomeniscal homologue (UMH) is an obliquely oriented, fibrocartilaginous structure, that forms part of the ulnar collateral ligament complex (ULC).
The UMH is located between the ulnar styloid process and the triquetrum. It adheres to the ulnar joint capsule and merges with ...
The umbilical artery gives rise to both a nonfunctional remnant of the fetal circulation, and an active vessel giving supply to the bladder. In the adult the obliterated area of the vessel is identifiable as the medial umbilical ligament and the patent segment is the superior vesical artery.
The umbilical cord is a fetal organ and connects the placenta to the uterus and is a vital passage for nutrients, oxygen and waste products to and from the fetus.
The umbilical cord inserts into the centre of the placental bulk and into the fetus at the umbilicus. Variations in insertion can o...
Uncovertebral joints, also called Luschka’s joints, are seen bilaterally between adjacent cervical vertebrae, identified by the cat ear shaped uncinate processes of the C3-7 vertebrae (C1 and C2 have no uncinate processes).
The articulation forms between the uncina...
The uncus is the innermost part of the anterior parahippocampal gyrus, part of mesial temporal lobe.
The uncus is the most anterior portion of the medial parahippocampal gyrus. It belongs to the limbic system. Housing the primary olfactory cortex, it is part of the olfactory sys...
Unfused spinous process, which is really failure of fusion of the neural arch, is a relatively common anatomical variant and is part of the spectrum of spina bifida occulta.
This should be differentiated from accessory ossicles of the spinous process, which appear after non-fusion of the secon...
A unicornuate uterus or unicornis unicollis is a type of Mullerian duct anomaly (class II) that is the second most commonly associated with miscarriages.
This type can account for ~10% (range 6-13%) of uterine anomalies and infertility is seen in ~12.5% (range 5-20%) of cases.
Upper limb anatomy encompasses the anatomy of the shoulder, arm, elbow, forearm, wrist and hand.
The upper subscapular nerve, also known as the short or superior subscapular nerve, arises from the posterior cord of the brachial plexus and supplies the subscapularis muscle.
The upper subscapular nerve branches from the posterior cord of the brachial plexus with fibres...
The upper T sign is one of the features useful in identifying the central sulcus on cross-sectional imaging.
It relies on identifying the superior frontal sulcus which intersects the precentral sulcus in a "T" junction, thus defining the precentral gyrus. The central sulcus is the next posterio...
The urachus is the fibrous vestigal remnant of the fetal allantois.
The lumen of the urachus usually obliterates after birth and it becomes the median umbilical ligament, a midline linear fibrous fold of parietal peritoneum, extending from the apex of the bladder to the umbilicus.
If the lumen...
The ureter is a paired fibromuscular tube that conveys urine from the kidneys in the abdomen to the bladder in the pelvis.
The ureter is 25-30 cm long and has three parts:
abdominal ureter: from the renal pelvis to the pelvic brim
pelvic ureter: from the pelvic brim to the bla...
Ureteral duplication is the most common congenital abnormality associated with the urinary tract, and occurs in ~1% of the population.
Duplication can occur unilaterally or bilaterally, and may be partial or complete:
partially duplicated ureters fuse into a single ureter proxim...
The ureteric bud (also known as the metanephrogenic diverticulum) is a protrusion of the mesonephric duct that appears during the embryological development of urogenital organs. It will eventually form the urinary collecting system (i.e. collecting tubes, calyces, renal pelvis, ureter) of the ki...
The urethra is the terminal segment of the genitourinary system. Because of vastly different anatomy between the sexes, male and female urethras are discussed separately:
The urinary bladder (more commonly just called the bladder) is a distal part of the urinary tract and is an extra-peritoneal structure located in the true pelvis.
The bladder has a triangular shape with a posterior base, an anterior apex and an inferior neck with two inferolatera...
The urinary system consists of the kidneys, ureter, bladder and urethra. With the exception of the urethra, this is equitable in both males and females. It spans the abdomen and pelvis, from the upper abdomen to the extreme pelvis, being inextricably linked with the genital system.
The U sign denotes the characteristic "U" shaped appearance of the subcentral gyrus which surrounds the inferolateral end of the central sulcus and abuts the lateral (Sylvian) fissure. It has been found, at least in one study, to be the most reliable anatomical feature to identify the central su...
Uterine agenesis is the extreme of Mullerian duct anomalies (Class I) where there is a complete absence of uterine tissue above the vagina.
The uterine agenesis-hypoplasia spectrum accounts for ~10-15% of all Müllerian duct anomalies.
The uterine artery is seen bilaterally and is a branch of the anterior division of the internal iliac artery.
It runs medially in the pelvis, within the base of the broad ligament, to the outer surface of the uterus. From lateral to medial it has a descending, transverse ...
Uterine duplication anomalies are a group of Müllerian duct anomalies where fusion of the Müllerian duct associated structures fail to some degree:
uterus didelphys: class III
bicornuate uterus: class IV (second commonest duplication anomaly)
septate uterus: class V (commonest duplication ano...
The uterine tube, also known as the fallopian tube, is a paired structure that bridges between each ovary and the uterus and functions to convey the mature ovum from the former to the latter. If conception occurs, it normally does so within the tube. It can be affected by a wide range of patholo...
The uterus is a hollow, thick-walled, muscular organ of the female reproductive tract that lies in the lesser pelvis.
The uterus has an inverted pear shape. In the adult, it measures about 7.5 cm in length, 5 cm wide at its upper part, and nearly 2.5 cm in thickness. It weighs ...
The uvea (also called the uveal layer or vascular tunic) is the middle three layers that make up the eye. It is the pigmented layer and its main function is of nutrition and gas exchange. It sits between the retina (innermost layer) and sclera.
It is traditionally split up into three anatomica...
The vagina is a midline fibromuscular tubular structure positioned in the female perineum extending superiorly to the cervix and uterus in the pelvis.
The vagina is 8-10 cm in length, extending posterosuperior from the vestibule through the urogenital diaphragm to the uterus. Th...
The vaginal artery is a branch of the anterior division of the internal iliac artery, and should not to be mistaken with the vaginal branch of the uterine artery. It is often considered to be a homolog of the inferior vesical artery, which is present only in males.
origin: anterior div...
The vagus nerve is the tenth cranial nerve and provides the bulk of the parasympathetic input to the gastrointestinal system and to the heart. It is a complex mixed sensory, motor and parasympathetic nerve.
The vagus nerve arises as multiple rootlets at the ...
The valvulae conniventes, also known as Kerckring folds, plicae circulares or just small bowel folds, are the mucosal folds of the small intestine, starting from the second part of the duodenum, they are large and thick at the jejunum and considerably decrease in size distally in the ileum to di...
Variant anatomy of the aortic arch occurs when there is failure of normal aortic development. It results in a number of heterogenous anomalies of the aorta and its branch vessels.
Normally, the aorta ascends in the superior mediastinum to the level of the sternal notch before arc...
Variation in hepatic arterial anatomy is seen in 40-45% of people. Classic branching of the common hepatic artery from the coeliac artery, and the proper hepatic artery into right and left hepatic arteries to supply the entire liver, is seen in 55-60% of the population.
In general, the common ...
There can be several variations in placental morphology. These include:
single lobed discoid placenta (single disc): most common scenario
bilobed placenta: two near equal size lobes
succenturiate lobe(s): one of more smaller accessory lobes
circumvallate placenta: rolled placental edges with...
Vascular anatomical variants are common.
SVC & IVC - variants
Intracranial arteries - variants
The vastus intermedius muscle is one of the 4 quadriceps muscles in the anterior compartment of the thigh. The others are the vastus medialis, the vests laterals, and the rectus femoris.
origin: upper two thirds of the anterior and lateral surfaces of the femur
The vastus lateralis is largest of the four quadriceps muscles in the anterior compartment of the thigh. The others are the rectus femoris, the vastus intermedius, and the vastus medialis.
lateral part of intertrochanteric line
margin of greater trochanter
The vastus medialis muscle is one of the four quadriceps muscles in the anterior compartment of the thigh. The others are the rectus femoris, the vastus intermedius, and the vastus lateralis.
medial part of intertrochanteric line
medial lip of the linea ...
The vein of Galen, also known as the great cerebral vein or great vein of Galen, is a short trunk formed by the union of the two internal cerebral veins and basal veins of Rosenthal. It lies in the quadrigeminal cistern. It curves backward and upward around the posterior border of the splenium o...
The vein of Labbé, also known as inferior anastomotic vein, is part of the superficial venous system of the brain.
The vein of Labbé is the largest channel that crosses the temporal lobe between the Sylvian fissure and the transverse sinus and connects the superficial middle cerebral vein to t...
The vein of Marshall, oblique vein of Marshall or the oblique vein of the left atrium is a small vein that descends on and drains the posterior wall of the left atrium. It drains directly into the coronary sinus at the same end as the great cardiac vein, marking the origin of the sinus.
Veins of Sappey are small veins around the falciform ligament that drain the venous blood from the anterior part of the abdominal wall directly into the liver. This flow dilutes the portal perfusion at these sites, causing hepatic pseudolesions.
The superior vein of Sappey drains...
The velum interpositum is a small membrane containing a potential space just above and anterior to the pineal gland which can become enlarged to form a cavum velum interpositum.
The velum interpositum is formed by an invagination of pia mater forming a triangular membrane the ap...
The vena caval foramen is one of the three major apertures in the diaphragm. It is the highest of the three and situated at the level of T8-9. It is quadrilateral and placed at the junction of the right and middle leaflets of the central tendon.
It transmits several structures between the thora...
The venae cordis minimae (smallest cardiac veins or thebesian veins) are a small group of valveless myocardial coronary veins within the walls of each of the 4 cardiac chambers that drain venous blood directly into each of the respective chambers. They are most frequent in the right atrium and t...
The venous drainage of the thoracic wall drains deoxygenated venous blood from the peripheries of the thoracic cage back into systemic circulation.
Anterior thoracic wall
Anterior intercostal veins
The anterior intercostal veins originate from the intercostal space just inferio...
The ventral horn of the spinal cord is one of the grey longitudinal columns found within the spinal cord. It contains the cell bodies of the lower motor neurones which have axons leaving via the ventral spinal roots on their way to innervate muscle fibres.
On transverse section o...
The ventricular system in the brain is composed of CSF-filled ventricles and their connecting foraminae. CSF is produced by ependymal cells which line the ventricles. They are continuous with the central spinal canal. Ventricles contain around 1/5 of normal adult CSF volume, which is around 20-2...
Evaluation of vermian lobulation is essential in assessment of the vermian maturity. MRI is a useful tool in assessment of the fetal posterior fossa.
Normal Vermian lobulation by weeks 1:
By 21 weeks - Prepyramidal fissure can be seen between the tuber and pyramis.
21 to 22 weeks - Preculmina...
The vermis (pl vermes) of the cerebellum is an unpaired medial structure which separates the cerebellar hemispheres. The neocerebellar posterior lobes join in the midline behind the primary fissure to separate the vermis into superior and inferior portions. The vermis can be furth...
The vertebra (plural: vertebrae) is the fundamental segmental unit of the vertebral column (also know as the spine).
Vertebrae, apart from those that are atypical, have a similar basic structure which can be described as an anterior vertebral body and a posterior neural (or verte...
The vertebral arteries (VA) are paired arteries, each arising from the respective subclavian artery and ascending in the neck to supply the posterior fossa and occipital lobes, as well as provide segmental vertebral and spinal column blood supply.
origin: branches off the 1st part of t...
Many vertebral levels are associated with key anatomical landmarks. Below is a summary of vertebral levels and associated internal or surface anatomy.
bifurcation of common carotid artery
trachea begins (end of la...
The vertebral venous plexus is a highly anastomotic network of valveless veins running along the entire length of the vertebral column from the foramen magnum to the sacral hiatus.
The vertebral venous plexus is comprised of three interconnected divisions:
internal vertebral ven...
C7, also called vertebra prominens, is the seventh cervical vertebra and looks like vertebrae C3-C6, but has some distinct features making it an atypical vertebrae. The name vertebra prominens arises from its long spinous process, which is easily palpable.
C7 possesses the stand...
The vertex is the midline bony landmark at the most superior part of the calvaria in the standard anatomical position, near the midpoint of the sagittal suture (i.e. between the bregma and lambda).
It is one of the skull landmarks, craniometric points for radiological or anthropological skull m...
The vertical fissure line commences in or near the costodiaphragmatic recess and heads superomedially towards the hilum and terminates before or at the horizontal fissure. More commonly occurs on the right and may indicate partial volume loss of the lower lobe 1.
The vertical muscle of the tongue is one of the 4 intrinsic muscles of the tongue which alters the shape of the tongue mass, being entirely confined to the tongue without an attachment outside the tongue (like the extrinsic muscles of the tongue).
The muscles fibres attach proxim...
Vesico-urachal diverticulum is one of the congenital urachal remnant abnormalities.
It is the proximal equivalent of a urachal umbilical sinus, representing a result of the failure of the urachus to close at the urinary bladder, forming an out-pouching of variable length from the...
The vestibular aqueduct is a structure of the inner ear being part of the osseous labyrinth. It contains the endolymphatic duct and sac. It normally has a diameter of ~1.5 mm (similar to the posterior semicircular canal) and runs from the vestibule in a transverse direction to the long axis of t...
The vestibule is an approximately 4 mm central chamber of the bony labyrinth. It is dominated by depressions housing the:
utricle (elliptical recess)
saccule (spherical recess)
basal end of the cochlear duct (cochlear recess)
The cribrose areas have perforations through which the nerve bundl...
Vickers ligament is a ligamentous structure connecting the lunate bone and TFCC to the distal radius and is seen in the vast majority of patients with Madelung deformity 1.
Release of this thickened ligament may help in improving symptoms associated with Madelung deformity 2.
There are two arteries passing through Vidian canal from the pterygopalatine fossa to the petrous portion of the ICA. One is a branch of the internal maxillary artery (itself a branch of the ECA) and the other is from the C2 segment of the ICA. It therefore forms one of the ICA to ECA anastamoses.
Vidian nerve, also known as the nerve of the pterygoid canal or nerve of the Vidian canal, is so named because of the canal in which is travels: the Vidian canal.
It is formed by the confluence of two nerves:
greater superfical petrosal nerve (from the geniculate ganglion of the facial nerve) ...
The visceral space is one of the infrahyoid deep spaces of the head and neck.
The visceral space extends from the hyoid bone to the superior mediastinum (level of aortic arch / T4), and is surrounded by the middle layers of the deep cervical fascia.
The visual system transmits visual information from the retina within the eyes to the primary visual cortex of the occipital lobe as well as the pretectal nuclei and superior colliculi of the midbrain.
Below the visual pathway is described from distal to proximal in a single hemi...
The vomer is one of the facial bones and forms the postero-inferior part of the bony nasal septum.
Occasionally the sphenoid sinus may pneumatise the vomer 2.
Waldeyer's ring is a ring of lymphoid tissue located in the nasopharynx and oropharynx at the entrance to the aerodigestive tract.
The structures composing this ring are:
palatine tonsils (also called the faucial tonsils)
adenoid tonsils (nasopharyngeal tonsils)
the lateral ba...
Wandering spleen is a rare condition in which the spleen migrates from its usual anatomical position, commonly to the lower abdomen or pelvis.
Wandering spleen is rare, with a reported incidence of <0.5%.
Diagnosis is most commonly made between ages 20 and 40 and is more common i...
Ward triangle refers to a radiolucent area between principle compressive, secondary compressive and primary tensile trabeculae in the neck of femur.
It should be differentiated from Babcock triangle.
History and etymology
Named for F O Ward after his original description of the region in 1838.
The Weigert-Meyer law describes the relationship of the upper and lower renal moieties in duplicated collecting systems to their drainage inferiorly.
With duplex kidney and complete ureteral duplication, the upper renal and lower renal moiety have their own ureters with each ...
Wernicke's area (Brodmann area 22) is an area of the posterior temporal lobe in the dominant hemisphere concerned with the receptive components of speech.
Wernicke's area is located in the superior temporal gyrus, posterior to the posterior commissure line.
It is boun...
Wharton jelly refers to the gelatinous substance within the umbilical cord.
Wharton jelly is derived from extra-embryonic mesoderm and is largely made up of mucopolysaccharides (hyaluronic acid and chondroitin sulphate) while containing smaller amounts of fibroblasts and macropha...
White matter tracts, also known as white matter fibres, are classified into three categories:
tracts connecting the cortex with other area in the CNS, e.g. deep nuclei, brainstem, cerebellum or spine
may be efferent (motor) or afferent tracts (sensory)
white matter tracts t...
The spinal cord has numeorus white matter tracts that ascend and descend in the peripheral substance of the cord. They can be divided by their location and function:
anterior corticospinal tract
medial longitudinal fasiculus
lateral spinothalamic t...
The Wolffian duct (also known as the mesonephric duct) is one of the paired embryogenic tubules that drain the primitive kidney (mesonephros) to the cloaca. In both the male and the female the Wolffian duct develops into the trigone of the urinary bladder.
In the female, in...
Wormian bones (a.k.a. intrasutural bones) is the name given to the additional small bones sometimes found between the cranial sutures of the bones of the skull vault, most commonly in relation to the lambdoid suture. Some reserve the term Wormian bones to just the intrasutural bones proximate to...
The xiphisternum (also known as the xiphoid process or simply the xiphoid) is the smallest of the three parts of the sternum (manubrium, body or gladiolus and xiphisternum). It arises from the inferior and posterior margin of the sternal body and projects inferiorly. It is a small cartilaginous ...
The zona orbicularis are circular fibres of the hip joint capsule and form a collar around the femoral neck. Though partly blended with the pubofemoral and ischiofemoral ligaments, these fibres are not directly attached to bone.
Zuckerkandl tubercle is a normal variant of the thyroid and may be mistaken for a thyroid nodule, mass or lymph node. It is a projection of normal thyroid tissue from the posterior aspect of the lateral lobes of the thyroid gland.
The tubercle is also an important surgical landmark due to its ...
The zygoma (or zygomatic bone) is an important facial bone which forms the prominence of the cheek. It is roughly quadrangular in shape.
Zygoma has three surfaces, five borders, and two processes.
anterolateral surface is convex, pierced at its orbital border by the zy...
The zygomatic arch is formed by the union of the temporal process of the zygomatic bone and the zygomatic process of the temporal bone at the zygomaticotemporal suture.
Le Fort type 3 fracture
zygomaticomaxillary complex fracture
The zygomatic nerve is a main branch of the maxillary division of the trigeminal nerve. It should not be confused with the zygomatic branch of the facial nerve.
The zygomatic nerve divides off the maxillary division just after emerging from the foramen rotundum to enter the ptery...
The zygomaticofacial foramen is a small foramen in the mid lateral surface of the zygomatic bone that transmits the zygomaticofacial nerve (a branch of the zygomatic nerve from the maxillary division of the trigeminal nerve) and zygomaticofacial vessels.
The zygomaticofacial nerve is the smaller of the two branches of the zygomatic nerve, from the maxillary division of the trigeminal nerve. It is sometimes referred to as the malar branch of the zygomatic nerve. It leaves the inferolateral aspect of the extraconal space of the orbit through the z...