Pain is an excessive stimulation of the sensory nerves, and in it all finer sensations are lost.  Thus, when a piece of hot iron burns the hand, the sensation is the same as when the iron is very cold, and extreme cold feels like intense heat.

317.  The Organ of Taste.  The sense of taste is located chiefly in the tongue, but may also be referred even to the regions of the fauces.  Taste, like touch, consists in a particular mode of nerve termination.

The tongue is a muscular organ covered with mucous membrane, and is richly supplied with blood-vessels and nerves.  By its complicated movements it is an important factor in chewing, in swallowing, and in articulate speech.  The surface of the tongue is covered with irregular projections, called papillae,—­fine hair-like processes, about 1/12 of an inch high.  Interspersed with these are the fungiform papillae.  These are shaped something like a mushroom, and may often be detected by their bright red points when the rest of the tongue is coated.

Towards the root of the tongue is another kind of papillae, the circumvallate, eight to fifteen in number, arranged in the form of the letter V, with the apex directed backwards.  These are so called because they consist of a fungiform papilla surrounded by a fold of mucous membrane, presenting the appearance of being walled around.

In many of the fungiform and most of the circumvallate papillae are peculiar structures called taste buds or taste goblets.  These exist in great numbers, and are believed to be connected with nerve fibers.  These taste buds are readily excited by savory substances, and transmit the impression along the connected nerve.

The tongue is supplied with sensory fibers by branches from the fifth and eighth pairs of cranial nerves.  The former confers taste on the front part of the tongue, and the latter on the back part.  Branches of the latter also pass to the soft palate and neighboring parts and confer taste on them.  The motor nerve of the tongue is the ninth pair, the hypoglossal.

[Illustration:  Fig. 125.—­The Tongue.

  A, epiglottis;
  B, glands at the base of tongue;
  C, tonsil;
  D, median circumvallate papilla,
  E, circumvallate papillae;
  F, filiform papillae;
  H, furrows on border of the tongue;
  K, fungiform papillae.
]

318.  The Sense of Taste.  The sense of taste is excited by stimulation of the mucous membrane of the tongue and of the palate, affecting the ends of the nerve fibers.  Taste is most acute in or near the circumvallate papillae.  The middle of the tongue is scarcely sensitive to taste, while the edges and the tip are, as a rule, highly sensitive.

Certain conditions are necessary that the sense of taste may be exercised.  First, the substance to be tasted must be in solution, or be soluble in the fluids of the mouth.  Insoluble substances are tasteless.  If we touch our tongue to a piece of rock crystal, there is a sensation of contact or cold, but no sense of taste.  On the other hand, when we bring the tongue in contact with a piece of rock salt, we experience the sensations of contact, coolness, and saline taste.