| head cap | A collapsed membranous vesicle that covers the anterior part of the nucleus of the spermatozoon, derived from the acrosomal granule; the carbohydrate-rich substance of the cap is associated with hydrolytic enzymes that aid in sperm penetration of the zona pellucida of the ovum. Synonym: head cap. (05 Mar 2000) |
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| head cavity | The cephalic region in the embryos of vertebrates containing the modified somites that give rise to the extrinsic eye muscles. (05 Mar 2000) |
| head-down tilt | Posture while lying with the head lower than the rest of the body. Extended time in this position is associated with temporary physiologic disturbances. (12 Dec 1998) |
| head-dropping test | A test used in the diagnosis of disease of the extrapyramidal or striatal system (e.g., parkinsonism, Wilson's disease); with the patient supine, relaxed, and his attention diverted, the examiner briskly lifts the patient's head with the right hand and then allows it to drop upon the palm of his left hand; the head of a normal person drops suddenly like a dead weight, whereas, in striatal disease the head falls slowly, gently, and almost hesitantly. (05 Mar 2000) |
| head fold | A ventral folding of the cephalic extremity in the embryonic disk, so that the brain lies rostrad to the mouth and pericardium. (05 Mar 2000) |
| head injuries | General or unspecified injuries involving the head. (12 Dec 1998) |
| head injuries, closed | Organic or functional damage resulting from trauma to the head where continuity of the scalp and mucous membranes is maintained. When brain injury results from closed head injuries, the primary cause is mechanical stretching and shearing of nerve fibres. Also common are focal intracranial lesions including haematomas and contusions. (12 Dec 1998) |
| head injury | Refers to a group of head injuries ranging from minor to major. Examples include scalp contusion, scalp haematoma, concussion, brain contusion, skull fracture, epidural haematoma, intracerebral haemorrhage, subarachnoid haemorrhage and subdural haematoma. Features shared by all head injuries (serious and nonserious) include: dizziness, nausea, vomiting, giddiness, sleepiness and headache. More serious features include: protracted vomiting, lethargy, difficulty waking up, loss of consciousness, seizure, confusion, change in mentation or coma. (27 Sep 1997) |
| head kidney | 1. The definitive excretory organ of primitive fishes. Synonym: head kidney. 2. In the embryos of higher vertebrates, a vestigial structure consisting of a series of tortuous tubules emptying into the cloaca by way of the primary nephric duct; in the human embryo, the pronephros is a very rudimentary and temporary structure, followed by the mesonephros and still later by the metanephros. Synonym: forekidney, primordial kidney. Origin: pro-+ G. Nephros, kidney (05 Mar 2000) |
| head lice | Small parasitic insects that can cause a itching red rash to the skin of the waist, shoulders, armpits, neck and pubic area. Commonly spread from person-to-person via close contact. The eggs (nits) of head lice can be mistaken for dandruff, but they are adherent to the hair shafts. Symptoms include itching, eggs visible on hair shafts and lice on hair, scalp or clothing. Treatment requires the use of a special prescription insecticide (Kwell) shampoo and/or lotion. (27 Sep 1997) |
| head mirror | A circular concave mirror attached to a head band, used to project a beam of light into a cavity, such as the nose or larynx, for purposes of examination and permitting binocular vision. (05 Mar 2000) |
| head movements | Voluntary or involuntary motion of head that may be relative to or independent of body; includes animals and humans. (12 Dec 1998) |
| head-nodding | Head movements associated with congenital nystagmus, spasmus nutans, and miner's nystagmus. Synonym: head tremors. (05 Mar 2000) |
| head nurse | A nurse administratively responsible for a designated hospital unit on a 24 hour basis. Synonym: charge nurse. (05 Mar 2000) |
| head of epididymis | The upper and larger extremity of the epididymis. Synonym: caput epididymidis, caput epididymis, globus major. (05 Mar 2000) |