Yesterday I suffered a fainting spell, which unfortunately sent me falling to the floor in my bathroom, doing some (superficial but dramatic) injury to my head and wiping out the toilet. (Men are currently at work fixing the property damage.) The fainting spell goes by the name vasovagal syncope in the technical terminology of medicine — an interesting term in all three of its parts.
Start with syncope, a word with a technical use in linguistics as well as medicine — both involving metaphors based ultimately on Greek. From NOAD2:
1 Medicine temporary loss of consciousness caused by a fall in blood pressure.
2 Grammar the omission of sounds or letters from within a word, e.g., when probably is pronounced |ˈpräblē|.
ORIGIN late Middle English: via late Latin from Greek sunkopē, from sun- ‘together’ + koptein ‘strike, cut off.’
Then vasovagal ‘relating to or denoting a temporary fall in blood pressure, with pallor, fainting, sweating, and nausea, caused by overactivity of the vagus nerve, esp. as a result of stress’ (NOAD2). (There are quite a few other causes, including defecation, especially massive defecation, as in diarrhea, which was my case; surprising numbers of people faint on or near the toilet.)
The combining form vaso- (from Latin vas ‘vessel’) denotes a vessel, usually a blood vessel. From Michael Quinion’s affixes site:
Most terms are medical ones relating to blood vessels and hence to blood pressure: vasoconstriction is the constriction of blood vessels, which increases blood pressure (the opposite is vasodilation); vasopressin (from pressor, something causing pressure) is a pituitary hormone which acts to increase blood pressure. However, vasectomy (Greek ektomē, excision), refers to the surgical cutting of the vas deferens, ducts which convey sperm from the testicles, typically as a means of sterilization.
That brings us to the vagus, or vagus nerve. From NOAD2:
Anatomy each of the tenth pair of cranial nerves, supplying the heart, lungs, upper digestive tract, and other organs of the chest and abdomen.
ORIGIN mid 19th cent.: from Latin [vagus ‘wandering, straying’; cf. vague]
The image seems to have been that these nerves wander over a considerable part of the middle body. When they are overactive, blood pressure falls, the blood supply to the brain is reduced, and dizziness or fainting can result: vasovagal syncope.