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Integumentary system
Integumentary system: the body's outer covering of skin, hair, nails, and glands (MeSH D034582). Largest organ, about 6 pounds; provides barrier, thermoregulation, vitamin D synthesis, and sensation.
Interaural time difference
Interaural time difference (ITD): the arrival-time difference of a sound at the two ears, the dominant cue for horizontal sound localization at low frequencies, encoded by coincidence-detecting MSO neurons, with human sensitivity near 10 microseconds and a maximum near 763 microseconds.
Intercostal nerves
Intercostal nerves (nervi intercostales) are the anterior rami of thoracic spinal nerves T1-T11 running in the costal grooves; T7-T11 continue as thoracoabdominal nerves, T12 is the subcostal nerve, and T2 gives the intercostobrachial nerve.
InterHeart study
The INTERHEART study was a case-control study of first heart attacks across 52 countries, finding that nine modifiable risk factors account for more than 90% of heart attack risk worldwide.
Intermittent claudication
Intermittent claudication, also called claudicatio intermittens, is exercise-induced leg pain relieved by rest and the most common symptom of peripheral artery disease; treated with supervised exercise, cilostazol, and selective revascularization.
Internal capsule
The internal capsule (capsula interna) is a compact band of white matter in each cerebral hemisphere where a small stroke can paralyze the opposite side of the body.
Internal carotid artery
Internal carotid artery (ICA): terminal branch of the common carotid artery supplying the brain and eye; seven segments C1–C7, first cerebral artery to form embryologically (day 24), ends as anterior and middle cerebral arteries.
Internal iliac artery
Internal iliac artery (hypogastric artery), terminal branch of the common iliac artery supplying the pelvis; divides into anterior and posterior trunks, mean length 39.95 mm, diameter 6.86 mm, with Adachi Type I branching in about 56% of people.
Internal jugular vein
Internal jugular vein (vena jugularis interna): paired carotid-sheath vein draining skull, brain, face, and neck to the brachiocephalic vein; JVP window and central access site with higher infection but fewer malpositions than subclavian.
Internal thoracic artery
The internal thoracic artery, also called the internal mammary artery, is a paired artery from the subclavian artery that supplies the anterior chest and upper abdominal walls.
Internal thoracic vein
Internal thoracic vein (vena thoracica interna, internal mammary vein): paired chest wall vein draining breast, pericardium, diaphragm, and anterior thorax into the brachiocephalic veins; often double, with xiphoid venous arch, used in flap reconstruction.
International Brain Research Organization
The International Brain Research Organization (IBRO), founded in 1961 with UNESCO support, is a global federation of neuroscience societies that funds training, exchanges, and collaborative research, especially in economically disadvantaged countries.
International Spinal Cord Society
International Spinal Cord Society (ISCoS), formerly International Medical Society of Paraplegia, founded 1961 by Ludwig Guttmann at Stoke Mandeville, renamed 2001; professional society for spinal cord injury care, research, and the Annual Scientific Meeting.
Interneuron
Interneuron: a neuron that is neither motor nor sensory; commonly, a local GABAergic inhibitory cell of the cortex and hippocampus, about 20% of cortical neurons, with PV, SST, and 5HT3aR subgroups and links to epilepsy, schizophrenia, and autism.
Interphalangeal joints of the hand
Interphalangeal joints of the hand (articulationes interphalangeales manus): nine synovial hinge joints, four PIP and four DIP plus the thumb IP joint, allowing flexion and extension; PIP provides 85% of finger grasping capability.
Interplexiform cell
Interplexiform cell (IPC), a retinal amacrine-cell subtype or separate class with processes in both inner and outer plexiform layers; coined by Gallego 1971; carries one-way centrifugal feedback from inner to outer retina via dopaminergic, GABAergic, or glycinergic synapses.
Interventricular septum
Interventricular septum (septum interventriculare): the muscular and membranous wall separating the heart's left and right ventricles; carries the bundle of His, supplied mainly by the LAD, and the site of ventricular septal defects.
Intervertebral disc
Intervertebral disc (IVD): fibrocartilaginous joint between vertebral bodies; 23 discs in the adult spine, each with nucleus pulposus, annulus fibrosus, and cartilaginous endplates; nucleus derives from the notochord; disc is avascular and prone to early degeneration.
Intervertebral foramen
Intervertebral foramen (foramen intervertebrale, neural foramen): opening between adjacent vertebrae linking the spinal canal to the extraspinal region; bounded by pedicles, disc, and facet capsule; transmits spinal nerves and vessels; narrowing causes foraminal stenosis.
Intestinal Barrier Function
Intestinal barrier function is the intestinal wall's selective defense against bacteria and toxic molecules while allowing passage of nutrients, electrolytes, and water; its measurable correlate is intestinal permeability.
Intestinal villus
An intestinal villus (plural villi) is one of the finger-like projections of the small intestine that absorb nutrients, amplifying the gut's surface area about 6.5-fold; their loss marks celiac disease.
Intimate part
Intimate part, also called private parts, is a euphemism for the genitals: vulva, vagina, penis, scrotum, and perineum. Covers anatomy, taboo vocabulary, and why precise naming matters in health care.
Intracranial granuloma
Intracranial granuloma (ICD-11 1D04.1) is a focal intracranial mass of macrophages and giant cells caused by infection (tuberculoma, fungal, parasitic), sarcoidosis, or foreign material; diagnosed by CT/MRI and biopsy.
Intracranial pressure
Intracranial pressure (ICP), also called intracranial hypertension when elevated, is the pressure inside the rigid skull from brain, blood, and CSF. Normal 7–15 mm Hg; above 20–25 mm Hg pathological, treated with osmotherapy, drainage, or craniectomy.
Intracranial space-occupying lesion
Intracranial space-occupying lesion (ICD-11 MB71.0, ICD-10-CM R90.0), also called intracranial mass lesion: any discrete intracranial lesion causing mass effect, mainly tumor, hemorrhage, abscess, or infarction, evaluated chiefly by contrast-enhanced MRI.
Intracranial vascular malformation
Intracranial vascular malformation (ICD-11 8B22.4): abnormal brain blood vessels including AVMs, dural fistulas, cavernous malformations, venous anomalies, and telangiectasias, differing in hemorrhage risk and treatment.
Intrafusal muscle fiber
Intrafusal muscle fiber: the small stretch-sensing fibers inside muscle spindles, in three types (bag1, bag2, chain), innervated by Ia/II afferents and dynamic/static gamma motoneurons for proprioception.
Intraocular medulloepithelioma
Intraocular medulloepithelioma, also called diktyoma, is a rare childhood embryonal tumor of the non-pigmented ciliary epithelium, the second most common pediatric intraocular tumor after retinoblastoma, linked to DICER1 mutations and treated mainly by enucleation.
Intraocular pressure
Intraocular pressure (IOP), also called eye tension, is the fluid pressure in the eye's anterior chamber, set by aqueous humor production and outflow; measured by tonometry, chiefly Goldmann applanation, and the main modifiable glaucoma risk factor.
Intraosseous venous malformation
Intraosseous venous malformation (IOVM) is a non-neoplastic venous vascular malformation composed of dilated, ectatic venous channels within bone. Classified as a low-flow malformation, it frequently affects the skull and facial skeleton.