Medical terms, defined.

The infraspinatus is a rotator cuff muscle that originates from the infraspinous fossa of the scapula and inserts on the middle facet of the greater tubercle of the humerus. It is the main external (lateral) rotator of the shoulder, inne...

The inner cell mass (embryoblast) is the cluster of cells within the blastocyst that gives rise to the embryo proper (epiblast and hypoblast) and several extraembryonic structures (amnion, yolk sac, allantois, and extraembryonic mesoderm...

The cytotrophoblast is the inner, mitotically active, mononuclear layer of the trophoblast. It proliferates and fuses to form the outer multinucleated syncytiotrophoblast, and it also gives rise to extravillous trophoblast that invades t...

The syncytiotrophoblast is the outer, multinucleated, terminally differentiated layer of the trophoblast that forms by fusion of underlying cytotrophoblast cells. It invades the endometrium for implantation, secretes hCG (maintaining the...

The ACL is an intra-articular, extrasynovial knee ligament running from the posteromedial lateral femoral condyle to the anterior intercondylar tibia. It prevents anterior tibial translation (and limits internal rotation/hyperextension)....

The ACL (anterior cruciate ligament) is an intra-articular knee ligament that prevents anterior translation of the tibia on the femur. It is the most commonly injured knee ligament, typically by a non-contact pivoting or deceleration inj...

PCL (posterior cruciate ligament) is an intra-articular, extrasynovial knee ligament that prevents posterior translation of the tibia on the femur. It is most commonly injured by a direct blow to the anterior proximal tibia (e.g., dashbo...

The PCL is an intra-articular, extrasynovial knee ligament that prevents posterior translation of the tibia on the femur. It runs from the lateral surface of the medial femoral condyle to the posterior intercondylar area of the tibia. It...

The medial collateral ligament (MCL, tibial collateral ligament) is a broad band on the medial knee that resists valgus stress. It is the most commonly injured knee ligament, typically from a lateral blow to the knee (valgus force). Inju...

LCL most commonly = lateral collateral ligament of the knee (fibular collateral ligament), which resists varus stress. It runs from the lateral femoral epicondyle to the fibular head. Injury is from a medial blow to the knee and is often...

The lateral collateral ligament (LCL, fibular collateral ligament) is an extracapsular band of the knee running from the lateral femoral epicondyle to the fibular head. It resists varus stress and, with the posterolateral corner, helps r...

The tarsal tunnel is a fibro-osseous passage on the medial ankle, posterior and inferior to the medial malleolus, bounded by the flexor retinaculum. It transmits the tibial nerve and tendons of tibialis posterior, flexor digitorum longus...

The menisci are C-shaped fibrocartilaginous wedges (medial and lateral) in the knee that deepen the tibial plateau, absorb shock, and distribute load. The medial meniscus is injured more often because it is less mobile and is attached to...

The lateral meniscus is a C-shaped (nearly O-shaped) fibrocartilaginous structure between the lateral femoral condyle and lateral tibial plateau. It is smaller, more circular, and more mobile than the medial meniscus, so it is injured le...

The nucleus pulposus is the gelatinous, water-rich central core of the intervertebral disc, remnant of the embryonic notochord. It acts as a shock absorber, and when it herniates through a torn annulus fibrosus it causes radiculopathy, c...

The annulus fibrosus is the tough outer ring of an intervertebral disc, made of concentric lamellae of type I collagen that surround and contain the gelatinous nucleus pulposus. Tears or weakening of it, most often posterolaterally, allo...

MCL most commonly means medial collateral ligament of the knee, which resists valgus stress and is injured by a lateral blow to the knee. On boards it can also mean mantle cell lymphoma (t(11;14), cyclin D1 overexpression, CD5+/CD23−) or...

The medial meniscus is a C-shaped fibrocartilaginous structure on the medial tibial plateau that deepens the articular surface, absorbs shock, and aids stability. It is less mobile than the lateral meniscus because it attaches firmly to...

The thoracic outlet is the anatomic passageway at the base of the neck between the thoracic cavity and the upper limb/axilla, through which the subclavian artery, subclavian vein, and brachial plexus (C5–T1) pass. Compression of these st...

The intervertebral disc is a fibrocartilaginous joint (symphysis) between adjacent vertebral bodies, made of a central gelatinous nucleus pulposus surrounded by a fibrous annulus fibrosus. It acts as a shock absorber and allows spinal fl...

The cubital tunnel is a fibro-osseous passage on the posteromedial elbow, between the medial epicondyle and olecranon, through which the ulnar nerve travels. Compression or traction there causes cubital tunnel syndrome, the second most c...

The cauda equina ("horse's tail") is the bundle of lumbosacral nerve roots (L2–S5 and the coccygeal nerve) that descends within the lumbar cistern below the conus medullaris (adult spinal cord ends ~L1–L2). Cauda equina syndrome (CES) is...

The conus medullaris is the tapered, cone-shaped distal end of the spinal cord, ending at approximately L1-L2 in adults (L3 in newborns). Lesions cause conus medullaris syndrome: early, sudden-onset bilateral symptoms with saddle anesthe...

The posterior (dorsal) horn of the spinal cord gray matter is the sensory-receiving region. It receives afferent input from primary sensory neurons (via dorsal roots) carrying pain, temperature, touch, and proprioception, and it contains...

The lateral horn is the intermediolateral cell column of the spinal cord gray matter at T1–L2 (L3 in some texts), containing cell bodies of preganglionic sympathetic neurons. It is present only at T1–L2 and forms the spinal origin of the...

A tendon sheath is a double-walled, fluid-filled synovial tube that surrounds tendons where they pass under retinacula, through osteofibrous tunnels, or across joints (e.g., hand, wrist, foot). It reduces friction and allows smooth glidi...

The dorsal column-medial lemniscus (DCML) pathway carries fine (discriminative) touch, vibration, two-point discrimination, and conscious proprioception from the body to the contralateral somatosensory cortex. Fibers ascend ipsilaterally...

The corticospinal tract is the major descending motor pathway for voluntary movement, especially fine, skilled movements of the distal limbs. Upper motor neurons in the motor cortex (primarily the precentral gyrus) descend through the in...

The endosteum is a thin, single-cell-thick connective tissue membrane lining the inner surfaces of bone: the medullary cavity, trabeculae of spongy bone, Haversian (osteonal) canals, and Volkmann canals. It contains osteoprogenitor cells...

A bursa is a flat, synovial fluid-filled sac lined by synovial membrane that reduces friction between moving structures (bone, tendon, muscle, skin). Inflammation (bursitis) presents with localized pain, swelling, and tenderness, commonl...

The periosteum is a dense connective tissue membrane covering the outer surface of bone (except at joint surfaces covered by articular cartilage and at sesamoid bones/tendon insertions). It is richly innervated and vascularized, provides...

The diaphysis is the shaft of a long bone, located between the proximal and distal epiphyses. It consists of thick cortical (compact) bone surrounding a central medullary cavity containing bone marrow, and it is the primary site of ossif...

Trabecular (cancellous/spongy) bone is the porous, lattice-like inner bone found in vertebral bodies, pelvis, and epiphyses/metaphyses of long bones. It makes up ~20% of skeletal mass but has ~80% of bone surface area, so it has higher m...

The epiphyseal (growth) plate is a hyaline cartilage disc between the epiphysis and metaphysis of long bones where longitudinal growth occurs via endochondral ossification. It is the weakest point of the pediatric skeleton, so it is pron...

Cancellous (trabecular/spongy) bone is the porous, lattice-like inner bone found in epiphyses and metaphyses of long bones and in vertebral bodies, pelvis, and flat bones. It makes up ~20% of skeletal mass but has high surface area and m...

Cortical (compact) bone is the dense, outer shell of bone making up ~80% of skeletal mass. It provides mechanical strength and protection, and is organized into osteons (Haversian systems) with lamellar bone. It has a slower turnover tha...

The metaphysis is the wide, flared portion of a developing long bone located between the epiphysis (growth-plate end) and the diaphysis (shaft). It lies adjacent to the physis (growth plate) and is the site of active bone growth and remo...

The Haversian system (osteon) is the basic structural unit of compact (cortical) lamellar bone, consisting of concentric bone lamellae surrounding a central Haversian canal that carries blood vessels, nerves, and lymphatics. Osteocytes i...

The epiphysis is the end portion of a long bone, separated from the diaphysis (shaft) by the physis (growth plate) during childhood. It is made of cancellous bone covered by articular cartilage and is the site of secondary ossification c...

A Haversian (central) canal is a longitudinal channel running through the center of an osteon (Haversian system) in compact (cortical) bone. It contains blood vessels, nerves, and loose connective tissue that supply osteocytes via canali...

An osteon (Haversian system) is the fundamental structural and functional unit of compact (cortical) lamellar bone, consisting of concentric lamellae surrounding a central Haversian canal containing blood vessels, nerves, and lymphatics....

Volkmann canals (perforating canals) are channels in compact bone that run perpendicular/oblique to the long axis of the bone, connecting adjacent Haversian (central) canals with each other and with the periosteal and endosteal (marrow)...

The foramen spinosum is a small opening in the greater wing of the sphenoid bone, posterolateral to the foramen ovale, that connects the middle cranial fossa to the infratemporal fossa. It transmits the middle meningeal artery and vein a...

T-tubules (transverse tubules) are invaginations of the sarcolemma that penetrate into skeletal and cardiac muscle fibers, rapidly conducting action potentials to the interior of the cell. They contain voltage-sensing L-type Ca2+ channel...

The jugular foramen is a large opening at the skull base between the temporal bone (petrous part) and occipital bone, transmitting CN IX, X, and XI plus the internal jugular vein (sigmoid sinus continuation) and meningeal branches. Lesio...

Meckel cartilage is the cartilaginous skeleton of the 1st pharyngeal (mandibular) arch. It serves as a template for mandible formation (via intramembranous ossification around it) and gives rise to the malleus and incus, the sphenomandib...

The foramen rotundum is a round opening in the greater wing of the sphenoid bone that connects the middle cranial fossa to the pterygopalatine fossa. It transmits the maxillary nerve (CN V2), the sensory branch of the trigeminal nerve su...

The internal auditory meatus (internal acoustic meatus/canal) is a canal in the petrous part of the temporal bone, running from the posterior cranial fossa to the inner ear. It transmits the facial nerve (CN VII), vestibulocochlear nerve...

The submandibular gland is a paired major salivary gland located in the submandibular triangle, producing ~70% of resting saliva (mixed serous > mucous). It is innervated by parasympathetic fibers from the facial nerve (CN VII) via the c...

The sublingual gland is the smallest of the three major paired salivary glands, located in the floor of the mouth beneath the tongue. It produces predominantly mucous saliva (mixed, mostly mucous), is innervated by parasympathetic fibers...