An azygos anterior cerebral artery is an uncommon to rare variant of the circle of Willis where the two A1 segments of the anterior cerebral artery (ACA) join to form a single trunk. As a result, there is no anterior communicating artery. This organization is similar to that seen in lower primat...
Azygos continuation of the inferior vena cava (also known as the absence of the hepatic segment of the IVC with azygos continuation) is an uncommon vascular anomaly and is a cause of a dilated azygos vein.
Spelling it "azygous" when referring to the vein is incorrect, regardless of...
An azygos lobe is created when a laterally displaced azygos vein creates a deep pleural fissure into the apical segment of the right upper lobe during embryological development. It is a normal anatomic variant of the right upper lobe due to the invagination of the azygos vein and pleura during d...
The azygos vein is a unilateral vessel that ascends in the thorax to the right of the vertebral column, carrying deoxygenated blood from the posterior chest and abdominal walls. It forms part of the azygos venous system.
The spelling azygous when referring to the vein is incorrect,...
The azygos (venous) system is a collective term given to the H-shaped configuration of the azygos, hemiazygos, accessory hemiazygos veins and left superior intercostal vein.
It is responsible for draining the thoracic wall and upper lumbar region via the lumbar veins and posterior intercostal v...
Babcock triangle is a relatively radiolucent triangular area seen in the subcapital region of the femoral neck on an anteroposterior radiograph of the hip. In this region, the trabeculae are loosely arranged and surrounded by more radiodense normal bony trabeculae groups. It may be the initial s...
A ball and socket ankle joint is a variant affecting the ankle where there is a rounded or spherical configuration to the talar dome with the corresponding concavity of the tibial plafond. The distal fibula may or may not be involved.
The etiology has been debated with two theories p...
Ball and socket joints are a type of synovial joint where the spheroid articular surface of one bone sits within a cup-like depression of another bone.
The ball and socket configuration allows for movement with 3 degrees of freedom, which is more than any other type of synovial joint...
The band of Giacomini, also known as the tail of the dentate gyrus or limbus Giacomini, is the anteromedial continuation of the dentate gyrus, which in turn is part of the hippocampal formation, along with the hippocampus, subicular complex and entorhinal cortex. It is an important anatomical la...
Barrow caroticocavernous fistula classification divides caroticocavernous fistulas into direct (type A) or indirect (types B-D). This classification was proposed by Barrow et al. in 1985 1 and at the time of writing (mid-2016) remains the most widely used system for describing caroticocavernous ...
The Bartholin glands, also known as the greater vestibular glands (or vulvovaginal glands) are paired pea-sized structures, lying on either side of the vaginal opening, and are homologous to the bulbourethral (Cowper) glands in the male. They form part of the vulva.
These glands ...
The basal ganglia are a group of grey matter nuclei in the deep aspects of the brain that is interconnected with the cerebral cortex, thalami and brainstem.
In a strict anatomical sense, it contains three paired nuclei that together comprise the corpus striatum:
The basal lamella, also known as basal lamella of the middle turbinate, is an osseous lamella that separates the anterior from the posterior ethmoid sinuses 1.
Internal anterior to posterior partitions of the ethmoid sinuses are called basal lamellae. According to a concept propose...
The basal nucleus of Meynert, also known as the nucleus basalis of Meynert, is formed by a group of cells that lies in the substantia innominata. It is rich in acetylcholine and its degeneration has been correlated to Alzheimer disease. It lies anterior to the anterior commissure.
The basal plate is a synonym for the maternal side of the placenta. The fetal side of the placenta is termed the chorionic plate.
Some pathologies and processes are localized to the basal plate, and evaluation of the basal plate is a part of placental grading.
The basal veins, also known as the veins of Rosenthal, are paired, paramedian veins which originate on the medial surface of the temporal lobe and run posteriorly and medially. Each vein passes lateral to the midbrain through the ambient cistern to drain into the vein of Galen with the internal ...
The base of the skull (or skull base) forms the floor of the cranial cavity and separates the brain from the structures of the neck and face.
The base of the skull is a bony diaphragm composed of a number of bones - from anterior to posterior:
Basilar artery fenestration (or more simply, basilar fenestration) is the most common intracranial arterial fenestration. It refers to duplication of a portion of the artery, thought to occur due to failed fusion of plexiform primitive longitudinal neural arteries 4. Its reported prevalence is h...
The basilar venous plexus lies between the endosteal and visceral layers of the dura on the inner surface of the clivus and connects numerous regional venous structures:
cavernous sinuses (superolaterally)
superior petrosal sinuses (superolaterally)
intercavernous sinuses (superio...
The basilic vein, along with the cephalic vein, is one of the primary superficial veins that drain the upper limb 1. It courses through both the forearm and arm, and contributes to the formation of the axillary vein.
origin: ulnar aspect of the superficial venous network of the dorsum ...
The basion is the median (midline) point of the anterior margin of the foramen magnum. The apical ligament attaches to it.
It is one of the skull landmarks, craniometric points for radiological or anthropological skull measurement.
Various lines and measurements using the ...
The basis pontis, or basilar/basal part of the pons, refers to the ventral portion of the pons, which contains white matter fibers of the corticospinal tract.
lacunar infarct in this region may result in pure motor hemiparesis, ataxic hemiparesis, or dysarthria-clumsy hand sy...
The Basle Nomina Anatomica (BNA) (a.k.a. Basiliensia Nomina Anatomica) is one of several anatomical nomenclature standards no longer in use 1-3. The Basle Nomina Anatomica was published in 1895 in Switzerland 1, but not adopted without controversy or uniformly even in neighboring countries 2. Th...
Bathrocephaly, also known as bathrocephalic occiputs, is a normal variation in skull shape, caused by an outward convex bulge of mid-portion of the occipital bone, often associated with a modification of the mendosal suture.
The true incidence of this disorder is unknown 1.
Batson venous plexus, also known as Batson veins, are a network of veins with no valves that connect deep pelvic veins draining the bladder, prostate, and rectum to the internal vertebral venous plexus 1. These veins are important because they are believed to provide a route for spread of pelvic...
Beaver tail liver, also known as a sliver of liver, is a variant of hepatic morphology where an elongated left liver lobe extends laterally to contact and often surround the spleen. It is more common in females. The parenchyma is normal and therefore has the same risks of hepatic pathology as th...
Betz cells are pyramidal cell neurons located within the fifth layer of the primary motor cortex. They are some of the largest in the central nervous system, sometimes reaching 100 µm in diameter and send their axons down the corticospinal tracts to the anterior horn cells of the spinal cord.
The biceps brachii muscle (also known simply as biceps) is a two-headed muscle in the anterior compartment of the arm that flexes at the elbow and supinates the forearm.
short head: coracoid process of the scapula
long head: supraglenoid tubercle of the scapula
The biceps femoris is one of the large muscles in the posterior compartment of the thigh and a component of the hamstrings. It has a long and a short head, each with different functions and innervation.
Its medial border forms the superolateral border of the popliteal fossa.
The biceps pulley is a capsuloligamentous complex that stabilizes the long head of biceps tendon within the bicipital groove. It comprises the coracohumeral ligament, superior glenohumeral ligament, and distal attachment of the subscapularis tendon. It is located within the rotator interval betw...
The bicipital groove (also known as the intertubercular sulcus or sulcus intertubercularis) is the indentation between the greater and lesser tuberosities of the humerus that lodges the biceps tendon.
The bicipital groove is typically 4-6 mm deep 1. It contains the tendon of the ...
The bicipitoradial bursa is located between the distal biceps brachii tendon and the tuberosity of the radius. The bursa partially or completely wraps around the biceps tendon. It ensures frictionless motion between the biceps tendon and the proximal radius during pronation and supination of the...
A bicornuate uterus is a type of uterine duplication anomaly. It can be classified as a class IV Mullerian duct anomaly.
Overall, congenital uterine anomalies occur in ~1.5% of females (range 0.1-3%). Bicornuate uteri are thought to represent ~25% (range 10-39%) of Mullerian duct ...
Bicuspid aortic valve (BAV) refers to a spectrum of deformed aortic valves with two functional leaflets or cusps which are often unequal in size.
They are most often congenital while an acquired bicuspid valve occurs when there is fibrous fusion between the right and left cusps of a pre-existin...
The median nerve may divide into two nerve bundles in the distal forearm and appear as a bifid median nerve in the carpal tunnel. It has an incidence of ~3%.
The median nerve usually divides into two or three branches after exiting the distal edge of the transverse carpal ligament that covers ...
The bifid postcentral gyrus sign, also known as the pars deflection sign, is a landmark useful for identifying the central sulcus on cross-sectional imaging.
The medial aspect of the postcentral gyrus splits in two before meeting the interhemispheric fissure. The two legs straddle the pars marg...
Bifid or forked or bifurcated rib is a congenital skeletal abnormality of the rib cage with the cleaved sternal end into two. They are thought to occur in ~0.2% of the population and there may be a female as well as right-sided predilection 2.
Bifid ribs can be seen ...
A bifid ureter, or ureter fissus, is an example of incomplete duplication of a duplex collecting system and is an anatomic variant.
Multiple seemingly unrelated terms for blind-ending bifid ureters are currently in use and there is no consensus on terminology in the literature. The...
The bifurcate ligament is one of the ligaments of the midtarsal joint, connecting the calcaneus with the navicular and the cuboid bone.
The bifurcate ligament arises from the anterior process of the calcaneus (distal to the origin of the extensor digitorum brevis). It consists of...
Various channels that collect bile from the hepatic parenchyma and transport it to the duodenum constitute the biliary tree.
By convention the biliary tree is divided into intra- and extra-hepatic bile ducts 1. There is significant variation in the biliary tree with the classical...
Bill bar is a bony anatomical landmark that divides the superior compartment of the internal acoustic meatus into an anterior and posterior compartment. Anterior to Bill bar, in the anterior superior quadrant, are the facial nerve (CN VII) and nervus intermedius, and posterior to it, in the post...
Bilobed testis, also known as incomplete unilateral polyorchidism, is a very rare variant in children.
The exact etiology is unknown but is thought to be a form of incomplete polyorchidism. It has been proposed that bilobed testis results from the incomplete division of the urogenita...
A bipartite medial cuneiform is an anatomical variant where there are two ossification centers involving the medial cuneiform. In many cases, the overall shape of the medial cuneiform is conserved, although the size of the two combined bones is larger than that of a normal medial cuneiform.
A bipartite patella (two-part patella) is a patella with an unfused accessory ossification center, typically at the superolateral aspect.
The superolateral accessory ossification center of the patella is usually present by 12 years of age and may persist into adult life. Bipartite...
A bipartite scaphoid is a rare example of a divided carpus. There is controversy whether this condition is congenital (i.e. normal variant) or post-traumatic. Bipartite scaphoids may be unilateral or bilateral.
Diagnostic criteria have been proposed 3:
no history of traumatic injury
Neuroanatomy of the bladder is complex, described here is a summary of the co-ordination of micturition.
The bladder acts as a reservoir normally storing 400-500 mL of urine under low pressure (<15 cmH2O) before voluntary voiding can occur at a socially-convenient time. Bladder filling and empt...
Block vertebra is a type of vertebral anomaly where there is a failure of separation of two or more adjacent vertebral bodies. It is an anatomic variant.
In a block vertebra, there is partial or complete fusion of adjacent vertebral bodies.
The blood-brain barrier (BBB) forms a physical resistance to the passage of lipophobic substances from cerebral capillaries into the brain and is a key reason why there is no CSF enhancement following intravenous contrast media on CT and MRI.
The blood-brain barrier is formed by ...
The blood supply of the meninges generally concerns the blood supply of the outer layer of dura mater rather than the inner layer of dura mater, arachnoid or pia mater which do not require a large blood supply. There are several arteries that supply the dura in the anterior, middle, and posterio...
Bochdalek's flower basket is the eponymous name for the incidental finding of protrusion of the choroid plexus through the foramina of Luschka. This is a relatively common finding.
It is an important normal variant to recognize as the presence of protruding calcified choroid tissue in the fourt...
The body of the sphenoid bone is the midline cubical portion of the sphenoid bone, hollowed by the sphenoid air sinuses.
The body has superior, inferior, anterior, posterior, and lateral surfaces.
The superior surface features:
ethmoidal spine: prominent spine that articulates...
Bone macroscopic structure allows a bone to be divided into regions based on position or morphology. This is important for a number of reasons including how growth may be affected by injury.
Bones can be separated into:
The bones in the lower limb can be divided into those within the thigh and leg (4) and those within the foot (26).
Bones of the thigh and leg
Bones of the foot
The bones in the upper limb are those that can be found within the pectoral girdle (3), arm (1) forearm (2), and hand (27). There are also a number of accessory ossicles, predominantly occurring at the wrist. Bones provide sites of attachment for skeletal muscles as well as a framework to neurov...
There are many types of bone within the body:
long bones (longer than they are wide)
short bones (not long bones)
sesamoid bones (for within a tendon)
irregular bones (don't fit into a category)
The majority of the bones of the appendicular skeleton are long bones. However, the ...
The bony orbit refers to the bones that constitute the margins of the orbits, that is the roof, medial and lateral walls and floor. The orbital margin or rim refers to the anterior circular margin of the orbit. The orbital apex refers to the posterior confluence of the orbit, where the optic can...
The bony pelvis is formed by the sacrum and coccyx and a pair of hip bones (os coxae or innominate bones), comprising the ischium, pubis and ilium and are part of the appendicular skeleton.
Its primary function is the transmission of forces from the axial skeleton to the lower limbs as well as...
The skeleton is the complete set of bones that make up a human. There are 206 bones in total which can be divided into:
The proximal femur has four major groups of trabeculae, distributing the compressive and tensile forces from the femoral head into the femoral diaphysis through the femoral neck. Together these trabeculae create the Ward triangle. The individual trabecular groups include:
principal compressive ...
Alain Bouthillier et al. described a seven segment internal carotid artery (ICA) classification system in 1996 1. It remains the most widely used system for describing ICA segments.
A helpful mnemonic for remembering ICA segments is:
C'mon Please Learn Carotid Clinical Organizing Classificati...
Bovine arch is the most common variant of the aortic arch and occurs when the brachiocephalic (innominate) artery shares a common origin with the left common carotid artery.
A bovine arch is apparent in ~15% (range 8-25%) of the population and is more common in individuals of African descent. ...
The Boyden classification of bronchi refers to the standard nomenclature used to describe bronchopulmonary segmental anatomy.
Each lung has 10 segments, however on the left, the first two segments share a common trunk and are hence B1/2. Also given the shared trunk on the left of the lower lobe...
The brachial artery is the main supply of arterial blood to the arm, forearm, and hand.
origin: continuation of the axillary artery distal to teres major
location: medial upper arm
supply: muscles of the arm, forearm, and hand
main branches: profunda brachii
terminal branches: rad...
The brachialis muscle (brachialis) is one of the three muscles of the anterior compartment of the arm. It is only involved in flexion at the elbow and therefore the strongest flexor at the elbow, compared with the biceps brachii which is also involved in supination because of its insertion on th...
The brachial plexus is a complex neural network formed by lower cervical and upper thoracic ventral nerve roots which supplies motor and sensory innervation to the upper limb and pectoral girdle. It is located in the neck extending into the axilla posterior to the clavicle.
The brachial vein is a component of the deep venous system of the upper limb. After forming from the radial and ulnar veins1, the brachial vein travels from the cubital fossa superiorly to become the axillary vein.
origin: union of the ulnar and radial veins in the cubital fossa1
The brachiocephalic trunk (BCT) (also known as the brachiocephalic artery, and previously as the innominate artery) is a major vessel that supplies the head, neck and right arm.
The brachiocephalic trunk is the first of the three main branches of the aortic arch, which originates...
The brachiocephalic veins, previously known as the innominate veins, are large paired valveless asymmetric veins that drain the head, neck, upper limbs and part of the thorax and mediastinum.
In the root of the neck, the internal jugular (IJV) and subclavian veins unite t...
The brachioradial artery, also known as a high origin of the radial artery, represents an anatomical variant of the arterial branching pattern of the upper limb 1,2. It represents an artery originating proximal to the cubital fossa that will go on to form the radial artery. The brachioradial art...
Brachioradialis muscle is a flexor at the elbow and works with biceps brachii and brachialis muscles. It is located in the superficial layer of the posterior compartment of the forearm and is particularly useful in elbow stabilization.
Despite the bulk of the muscle being visible from the anter...
The bracket sign of the pars marginalis, also known as the pars bracket sign, refers to the appearances of the superior most extent of the pars marginalis of the cingulate sulcus on axial imaging. It forms two roughly symmetric brackets, open anteriorly. The next sulcus anteriorly is the central...
The brain is the vital neurological organ composed of:
The brain is housed in the skull and bathed in cerebrospinal fluid. It is continuous with the cervical spinal cord at the cervicomedullary junction.
An understanding of brain arterial vascular territories is important in understanding stroke and complications from surgery and endovascular procedures.
Although one could be excused for thinking that within the brain, such a carefully organized organ, blood supply would be constant, the truth...
Brain development occurs from the three vesicles of the embryo's neural tube.
By approximately 4.5 to 5 menstrual weeks, the primitive neural plate has developed. The neural plate then divides into the neural crest and...
The brainstem is the most caudal part of the brain. It adjoins, is structurally continuous with the spinal cord and consists of the:
pons (part of the metencephalon)
medulla oblongata (myelencephalon)
The brainstem provides the main motor and sensory innervation to t...
The brainstem nuclei are the nuclei in the brainstem. These include:
cranial nerve nuclei
Useful mnemonics to remember the branches of the internal carotid artery is:
A VIP'S COMMA
Calming Voices Make Intra-Operative Surgery Pleasurable And Almost Memorable
A VIP'S COMMA
A: anterior choroidal artery (C7)
V: Vidian artery (C2)
I: inferolateral trunk (C4)
A useful mnemonic to remember the branches of the internal iliac artery is:
I Love Going Places In My Very Own Underwear!
I: iliolumbar artery
L: lateral sacral artery
G: gluteal (superior and inferior) arteries
P: (internal) pudendal artery
I: inferior vesical (vaginal in female...
A mnemonic for remembering the branches of the maxillary artery is:
DAM I AM Piss Drunk But Stupid Drunk I Prefer, Must Phone Alcoholics Anonymous
D: deep auricular artery
A: anterior tympanic artery
M: middle meningeal artery
I: inferior alveolar artery
A: accessory meningeal ar...
A useful mnemonic to remember the branches of the ophthalmic artery is:
D: dorsal nasal artery
R: (central) retinal artery
M: muscular artery
C: ciliary arteries (long, short and anterior)
L: lacrimal artery
E: ethmoidal arteries (anterior and posterior)
Mnemonics to remember the branches of the anterior division of the internal iliac artery are:
SOI VU MR PIG (it can be remembered as SO fourth (IV) U MR PIG)
Oranges Under Some Ice Might Peel Instantly
SOI VU MR PIG
S: superior vesical artery
O: obturator artery
IV: inferior ves...
Useful mnemonics to remember the four branches of the thoracoacromial artery are:
CAlifornia Police Department
Cadavers Are Dead People
B: breast (pectoral)
CAlifornia Police Department
The branchial (or pharyngeal) apparatus is the complex region in the developing embryo between the head and chest that develops in the fourth week and provides bilateral ridges and valleys that subsequently develop into numerous anatomic structures of the head, face, palate and anterior neck. Th...
The breast is an apocrine gland seen in both males and females. However, in females it has a specific function which is the production of milk for neonatal nutrition and immune function.
The breast has an inhomogeneous structure which is predominantly composed of adi...
Breast hypoplasia is a condition which is characterized by underdevelopment of the breast. Breast hypoplasia can be congenital or acquired.
congenital adrenal hyperplasia
The bregma is the midline bony landmark where the coronal and sagittal sutures meet, between the frontal and two parietal bones.
It is the anterior fontanelle in the neonate and closes in the second year 2 (typically around 18 months after birth).
It is one of the skull landmarks, craniometric...
A bridging bronchus is a rare congenital bronchial anomaly where there is an anomalous bronchus to the right lung arising from the left main bronchus. It has a high association with right upper lobe bronchus (pig bronchus) and congenital cardiac and vascular malformations, particularly a left pu...
Bridging of the sella turcica is the fusion of the anterior and posterior clinoid processes.
The prevalence of a sella turcica bridge in a healthy population is estimated to be ~4% (range 1.75-6 %) in anatomical and radiographic studies.
It has been reported to occur i...
The broad ligament is the lateral folds of the parietal peritoneum which reflect over the upper genital tract in females.
The broad ligament extends from the lateral aspect of the uterus to the lateral pelvic wall and can be divided into three main components - the mesosalpinx, m...
A mnemonic for the contents of the broad ligament is:
B: bundle (ovarian neurovascular bundle)
R: round ligament
O: ovarian ligament
A: artifacts (vestigial structures)
D: duct (oviduct)
Broca's area (Brodmann area 44) is an area of the lateral frontal lobe in the dominant hemisphere concerned with the production of speech.
Broca's area is located in the posterior inferior frontal gyrus (pars opercularis and pars triangularis) of the dominant hemisphere, anterior...
Brodmann areas are a system to divide the cerebral cortex according to cytoarchitectural organization, and are, despite controversy, still very widely used as a standardized nomenclature which is superimposed on the somewhat variable gyral and sulcal anatomy.
The classification relies on the f...
The bronchial arteries are the major supply of high-pressure oxygenated blood to the supporting structures of the lung, including the pulmonary arteries, yet they are responsible for only 1% of the lung blood flow overall.
Bronchial artery anatomy is variable, most commonly classified according...