tube._ Three ganglia, usually designated as the _superior,
middle_, and _inferior_, connect with the cervical and spinal nerves.
Their interlacing filaments are distributed to the muscular walls of the
larynx, pharynx, trachea, and esophagus, and also penetrate the _thyroid
gland_. The use of this gland is not accurately known. It is composed of
a soft, brown tissue, and consists of lobules contained in lobes of
larger size. It forms a spongy covering for the greater portion of the
larynx, and the first section of the trachea. That it is an important
organ, is evident from the fact that it receives four large arteries,
and filaments from two pairs of nerves.

The sympathetic ganglia of the chest correspond in number with the
terminations of the ribs, over which they are situated. Each ganglion
receives two filaments from the intercostal nerve, situated above it,
thus forming a double connection. The thoracic ganglia supply with motor
fibres that portion of the aorta which is above the diaphragm, the
esophagus, and the lungs.

In the abdomen the sympathetic centers are situated upon the _coeliac_
artery, and are termed, collectively, the _semilunar coeliac ganglion_.
Numerous inosculating branches radiate from this center and are called,
from the method of their distribution, the _solar plexus_. From this,
also, originate other plexi which are distributed to the stomach, liver,
kidneys, intestines, spleen, pancreas, supra-renal glands, and to the
organs of generation. Four other pairs of abdominal ganglia connected
with, the lumbar branches are united by filaments to form the semilunar
ganglion.

The sympathetic ganglia of the pelvis consist of five pairs, which are
situated upon the surface of the sacrum. At the extremity of the spinal
column this system terminates in a single knot, designated as the
_ganglion impar_.

Owing to the position of the sympathetic ganglia, deeply imbedded in the
tissues of the chest and abdomen, it is exceedingly difficult to subject
them to any satisfactory experiments. A few isolated facts form the
basis of all our knowledge concerning their functions. They give off
both motor and sensory filaments. The contraction of the _iris_ is one
of the most familiar examples of the action of the sympathetic system.

In the reflex actions of the nerves of special sense, the sensation is
transmitted through the cerebro-spinal system, and the motor impulse is
sent to the deep-seated muscles by th