As a result of a depraved condition of blood, the seeing-center itself may be unduly stimulated, and a person may see objects which appear real.  Thus in an attack of delirium tremens, the victim of alcoholic poisoning sees horrible and fantastic creatures.  The diseased brain refers them as usual to the external world; hence they appear real.  As the sufferer’s judgment is warped by the alcoholic liquor, he cannot correct the impressions, and is therefore deceived by them.

313.  Organs of Special Sense.  The organs of special sense, the means by which we are brought into relation with surrounding objects, are usually classed as five in number.  They are sometimes fancifully called “the five gateways of knowledge”—­the skin, the organ of touch; the tongue, of taste; the nose, of smell; the eye, of sight; and the ear, of hearing.

[Illustration:  Fig. 124.—­Magnified View of a Papilla of the Skin, with a Touch Corpuscle.]

314.  The Organ of Touch.  The organ of touch, or tactile sensibility, is the most widely extended of all the special senses, and perhaps the simplest.  It is certainly the most precise and certain in its results.  It is this sense to which we instinctively appeal to escape from the illusions into which the other senses may mislead us.  It has its seat in the skin all over the body, and in the mucous membrane of the nostrils.  All parts of the body, however, do not have this sense in an equal degree.

In Chapter IX. we learned that the superficial layers of the skin covers and dips in between the papillae.  We also learned that these papillae are richly provided with blood-vessels and sensory nerve fibers (sec. 234).  Now these nerve fibers terminate in a peculiar way in those parts of the body which are endowed with a very delicate sense of touch.  In every papilla are oval-shaped bodies about 1/300 of an inch long, around which the nerve fibers wind, and which they finally enter.  These are called touch-bodies, or tactile corpuscles, and are found in great numbers on the feet and toes, and more scantily in other places, as on the edges of the eyelids.

Again, many of the nerve fibers terminate in corpuscles, the largest about 1/20 of an inch long, called Pacinian corpuscles.  These are most numerous in the palm of the hand and the sole of the foot.  In the papillae of the red border of the lips the nerves end in capsules which enclose one or more fibers, and are called end-bulbs.

The great majority of the nerve fibers which supply the skin do not end in such well-defined organs.  They oftener divide into exceedingly delicate filaments, the terminations of which are traced with the greatest difficulty.

315.  The Sense of Touch.  Touch is a sensation of contact referred to the surface of the body.  It includes three things,—­the sense of contact, the sense of pressure, and the sense of heat and cold.

The sense of contact is the most important element in touch.  By it we learn of the form, size, and other properties of objects, as their smoothness and hardness.  As we all know, the sense of touch varies in different parts of the skin.  It is most acute where the outer skin is thinnest.  The tips of the fingers, the edges of the lips, and the tip of the tongue are the most sensitive parts.