Hemangioma:  An aggregation of multiple dilated blood vessels.
Hematoma:  A blood clot.
Hemianopia:  The loss of vision of one-half of the visual field.
Hemiplegia:  The paralysis of one side of the body.
Hemorrhage:  Bleeding due to the escape of blood from a blood vessel.
Herniated nucleus pulposus (HNP):  The extrusion of the central portion (nucleus) of an intervertebral disc through the outer cartilaginous ring (annulus). The herniated portion can compress the spinal cord or nerves in or exiting the spinal canal.
Hydrocephalus:  A condition, often congenital, marked by abnormal and excessive accumulation of cerebrospinal fluid in the cerebral ventricles. This dilates the ventricles and increases intracranial pressure. More about hydrocephalus.
Hydromyelia:  Expansion of the spinal cord due to increased size of the central canal of the cord, which is filled with cerebrospinal fluid.
Hyperesthesia:  Excessive sensibility to touch, pain, or other stimuli.
Hypertension:  High blood pressure.
Hypophysectomy:  Surgical removal of the hypophysis (pituitary gland).
Hypothalamus:  A collection of specialized nerve cells at the base of the brain that controls the anterior and posterior pituitary secretions, and is involved in other basic regulatory functions such as temperature control and attention.

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