(disk )

[L. discus, fr. Gr. diskos, a dish, quoit, discus]

A flat, round, platelike structure.

anisotropic d.A band .

articular d.A biconcave oval disk of fibrous connective tissue that separates the two joint cavities of the temporomand ibular joint on each side.

choked d.Papilledema.

dental d.A thin disk of paper (or other substance) used to abrade, cut, or polish teeth or dental appliances.

d. at risk An optic disk whose function is threatened by insufficient blood flow. On funduscopic examination, the optic nerve appears small and crowded, and the cup-to-disk ratio is small.

SEE: cup-to-disk ratio.

embryonic d.An oval disk of cells in the blastocyst of a mammal from which the embryo proper develops. Its lower layer (the endoderm) forms the roof of the yolk sac. Its upper layer (the ectoderm) forms the floor of the amniotic cavity. The primitive streak develops on the upper surface of the disk.

SEE: embryo for illus.

epiphyseal d.A disk of cartilage at the junction of the diaphysis and epiphyses of growing long bones. Cartilage synthesis provides for growth in length; the cartilage is eventually replaced by bone.

germinal d.A disk of cells on the surface of the yolk of a teloblastic egg from which the embryo develops. SYN: blastoderm.

herniated d.Rupture of the soft tissue that separates two vertebral bones into the spinal canal or adjacent spinal nerve roots. Herniation of intervertebral disks can cause back pain and , occasionally, loss of neurological function in the distribution of affected nerves.SYN: herniated intervertebral disk; lumbar disk prolapse; slipped disk.

SEE: herniation of nucleus pulposus for illus.

herniated intervertebral d.Herniated disk.

intercalated d.A modification of the cell membrane of adjacent cardiac muscle cells, consisting of extensive folds and intercellular junctions for electrical and mechanical linkage of contiguous cells.

SEE: illus.

intervertebral d.The fibrocartilaginous tissue between the vertebral bodies. The outer portion is the anulus fibrosus; the inner portion is the nucleus pulposus. The disk is a shock absorber, or cushion, and permits movement.

M d.M line.

Merkel d.

SEE: Merkel disk.

optic d.The area of the retina where the optic nerve enters. SYN: blind spot (1); optic nerve head.

Placido d.

SEE: Placido disk.

slipped d.A colloquial term for herniated disk.

SEE: herniated disk.

Z d.A thin, dark disk that transversely bisects the I band (isotropic band ) of a striated muscle fiber. The thin filaments, made primarily of actin, are attached to the Z disk; the area between the two Z disks is a sarcomere.