This system consists of two main trunks or cords of ganglia (nerve centers), extending from the base of the brain to the coccyx, and situated partly on the right and left sides and partly in front of the spinal column; and, further, of three great nerve plexuses and many smaller ganglia in the body cavities; and of numerous nerve fibers extending from these structures. The two cords converge above in a small ganglion in the brain, and below in the coccygeal ganglion in front of the coccyx.

In Fig. VI-B, to the left of the spinal column, is indicated one of the two cords of the involuntary nervous system. From it are seen to extend widespread ramifications of nerve fibers, which form the plexuses that are spread like spider webs over the digestive and the other organs in the body cavities; in the solar plexus they are joined by the vagus nerve of the voluntary system.

Fig. VI-C is a sketch indicating the two ganglionic cords of the involuntary system, converging below; running down between them is the spinal cord, terminating near the coccyx. On the sides are indicated the kidneys, topped by the adrenals.