erve
varies from about 5,000 to 100,000. Nerves can easily be identified in a
piece of lean beef, or even at the edge of a serious gash in one's own
flesh!

Bundles of sensory fibers constituting a sensory nerve root enter the
spinal cord on the posterior side through holes in the vertebrae. Similar
bundles of motor fibers in the form of a motor nerve root emerge from
the cord at the same level. Soon after their emergence from the cord,
these two nerves are wrapped together in the same sheath and proceed in
this way to the periphery of the body, where the sensory nerve usually
ends in a specialized _end-organ_ fitted to respond to some certain
stimulus from the outside world. The motor nerve ends in minute
filaments in the muscular organ which it governs. Both sensory and motor
nerves connect with fibers of like kind in the cord and these in turn
with the cortex, thus giving every part of the periphery direct
connection with the cortex.

[Illustration: 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