FIG. 12.—Schematic diagram showing association fibers connecting cortical centers with each
other.—After JAMES and STARR.The end-organs of the sensory nerves are nerve masses, some of them, as the
taste buds of the tongue, relatively simple; and others, as the eye or ear, very
complex. They are all alike in one particular; namely, that each is fitted for its
own particular work and can do no other. Thus the eye is the end-organ of sight,
and is a wonderfully complex arrangement of nerve structure combined with
refracting media, and arranged to respond to the rapid ether waves of light. The
ear has for its essential part the specialized endings of the auditory nerve, and is
fitted to respond to the waves carried to it in the air, giving the sensation of
sound. The end-organs of touch, found in greatest perfection in the finger tips,
are of several kinds, all very complicated in structure. And so on with each of
the senses. Each particular sense has some form of end-organ specially adapted
to respond to the kind of stimulus upon which its sensation depends, and each is
insensible to the stimuli of the others, much as the receiver of a telephone will
respond to the tones of our voice, but not to the touch of our fingers as will the
telegraph instrument, and vice versa. Thus the eye is not affected by sounds, nor
touch by light. Yet by means of all the senses together we are able to come in
contact with the material world in a variety of ways.
5. LOCALIZATION OF FUNCTION IN THE NERVOUS SYSTEM
Division of Labor.—Division of labor is the law in the organic world as in the
industrial. Animals of the lowest type, such as the amœba, do not have separate
organs for respiration, digestion, assimilation, elimination, etc., the one tissue