The Collaborative International Dictionary of English v.0.48:

Tripod \Tri"pod\, n. [L. tripus, -odis, Gr. ?; ? (see Tri-) +
   ?, ?, foot. See Foot, and cf. Tripos, Trivet.]
   1. Any utensil or vessel, as a stool, table, altar, caldron,
      etc., supported on three feet.
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   Note: On such, a stool, in the temple of Apollo at Delphi,
         the Pythian priestess sat while giving responses to
         those consulting the Delphic oracle.
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   2. A three-legged frame or stand, usually jointed at top, for
      supporting a theodolite, compass, telescope, camera, or
      other instrument.
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   Tripod of life, or Vital tripod (Physiol.), the three
      organs, the heart, lungs, and brain; -- so called because
      their united action is necessary to the maintenance of
      life.
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The Collaborative International Dictionary of English v.0.48:

Vital \Vi"tal\, a. [F., fr. L. vitalis, fr. vita life; akin to
   vivere to live. See Vivid.]
   1. Belonging or relating to life, either animal or vegetable;
      as, vital energies; vital functions; vital actions.
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   2. Contributing to life; necessary to, or supporting, life;
      as, vital blood.
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            Do the heavens afford him vital food? --Spenser.
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            And vital virtue infused, and vital warmth.
                                                  --Milton.
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   3. Containing life; living. "Spirits that live throughout,
      vital in every part." --Milton.
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   4. Being the seat of life; being that on which life depends;
      mortal.
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            The dart flew on, and pierced a vital part. --Pope.
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   5. Very necessary; highly important; essential.
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            A competence is vital to content.     --Young.
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   6. Capable of living; in a state to live; viable. [R.]
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            Pythagoras and Hippocrates . . . affirm the birth of
            the seventh month to be vital.        --Sir T.
                                                  Browne.
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   Vital air, oxygen gas; -- so called because essential to
      animal life. [Obs.]

   Vital capacity (Physiol.), the breathing capacity of the
      lungs; -- expressed by the number of cubic inches of air
      which can be forcibly exhaled after a full inspiration.

   Vital force. (Biol.) See under Force. The vital forces,
      according to Cope, are nerve force (neurism), growth force
      (bathmism), and thought force (phrenism), all under the
      direction and control of the vital principle. Apart from
      the phenomena of consciousness, vital actions no longer
      need to be considered as of a mysterious and unfathomable
      character, nor vital force as anything other than a form
      of physical energy derived from, and convertible into,
      other well-known forces of nature.

   Vital functions (Physiol.), those functions or actions of
      the body on which life is directly dependent, as the
      circulation of the blood, digestion, etc.

   Vital principle, an immaterial force, to which the
      functions peculiar to living beings are ascribed.

   Vital statistics, statistics respecting the duration of
      life, and the circumstances affecting its duration.

   Vital tripod. (Physiol.) See under Tripod.

   Vital vessels (Bot.), a name for latex tubes, now disused.
      See Latex.
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