Section 132. Besides the Great Mass of Brain and Spinal Cord

(cerebro-spinal axis), there is, on either side of the dorsal wall of the body cavity, a sympathetic nervous chain. The nerve fibres of this system, like the nerve fibres of invertebrates, are non-medullated. It may be seen as a greyish thread running close by the common carotid in the neck (sym., Figure 1); it then runs over the heads of the ribs in the thorax and close beside the dorsal aorta in the abdominal region. In the anterior region of the neck it dilates to form a superior cervical ganglion, and opposite the first rib it forms an inferior cervical ganglion. Thence, backwards, there is a ganglion on each sympathetic chain opposite each spinal nerve, and the two exchange fibres through a thread, the ramus communicans. To the sympathetic chain is delegated much of the routine work of reflex control of the bloodvessels and other viscera, which would otherwise fall upon the spinal cord.