ne pairs, which connect the spinal cord with different
parts of the trunk, with the upper, and with the lower extremities. Each
nerve joins the cord by two roots, these being named from their positions
the _ventral_, or anterior, root and the _dorsal_, or posterior, root. The
two roots blend together within the spinal cavity to form a single nerve
trunk, which passes out between the vertebrae. On the dorsal root of each
spinal nerve is a small ganglion which is named, from its position, the
_dorsal-root ganglion_. (Consult Figs. 133 and 135, and also Fig. 125.)

*Double Nature of Spinal Nerves.*--Charles Bell, in 1811, made the
remarkable discovery that each spinal nerve is double in function. He
found the portion connecting with the cord by the dorsal root to be
concerned in the _production of feeling_ and the portion connecting by the
ventral root to be concerned in the _production of motion_. In keeping
with these functions, the two divisions of the nerve are made up of
different kinds of fibers, as follows:

1. The dorsal-root divisions, of the fibers of di-axonic neurons, the
cell-bodies of which form the dorsal-root ganglia (Fig. 135).

2. The ventral-root divisions, of the fibers of mon-axonic neurons, the
cell-bodies of which are in the gray matter of the cord.

The first convey impulses to the cord and are called _afferent_
neurons;(102) the second convey impulses from the cord and are known as
_efferent_ neurons. Thus, by forming a part of the nerve pathways between
the skin and the brain, the dorsal divisions of these nerves aid in the
production of feeling; and by completing pathways to the muscles, the
ventral divisions aid in the production of motion (Figs. 129, 135, and
141).

[Fig. 135]

Fig. 135--*Connection of spinal nerves with the cord.* On the right is
shown a nerve pathway from the skin to the muscle. A division of this
pathway reaches the brain.

*The Cranial Nerves.*--From the under front surface of the brain, twelve
pairs of nerves emerge and pass to the head, neck, and upper portions of
the trunk. These, the cranial nerves, have names suggestive of their
function or distribution and, in addition, are given numbers which
indicate the order in which they leave the brain (Fig. 136). Unlike the
spinal nerves, the cranial nerves present great variety among themselves,
scarcely any two of them being alike in function or in their co