FIG. 11.—The projection fibers of the brain. I-IX, the first nine pairs of cranial nerves.It will help in understanding the peripheral system to remember that a nerve
consists of a bundle of neurone fibers each wrapped in its medullary sheath and
sheath of Schwann. Around this bundle of neurones, that is around the nerve, is
still another wrapping, silvery-white, called the neurilemma. The number of
fibers going to make up a nerve 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 vertebræ. 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.