The _sacral plexus_ supplies the organs of the pelvis, the thigh, and the leg. Its chief branches are the great sciatic, the largest nerve in the body, and the small sciatic, which go to the buttocks and thigh. The great sciatic runs down the back of the thigh and divides at the lower third of the thigh into the internal and external popliteal nerves, the former of which passes along the back of the thigh to the knee, where it becomes the posterior tibial, which in turn divides at the ankle into the internal and external plantar. The external popliteal descends along the outer side of the popliteal space and divides an inch below the head of the fibula into the anterior tibial, which supplies the flexors and skin of the ankle-joint, and the musculo-cutaneous, which sends branches to the skin of the lower leg and the dorsum of the foot. =The Sympathetic System.=--Joined to the cerebro-spinal system by intervening cords is the sympathetic system. This is made up of two series of ganglia, one on either side of the spinal column, connected by longitudinal bands and extending from the base of the skull to the coccyx. They do not form an independent nervous system, each ganglion, which seems to resemble the motor cells of the spinal cord, being connected by motor and sensory fibers with the cerebral system. The _sympathetic nerves_ are mostly gray, non-medullated fibers and are distributed to viscera, secreting glands, and blood-vessels, whose movements are involuntary and feelings obtuse. They form networks upon the heart and other viscera and send branches to the cranium to the organs of special sense. There are three main plexuses: The _solar plexus_ behind the stomach, which supplies the abdominal viscera; the _hypogastric plexus_ in front of the prominence of the sacrum, whose nerves go to the pelvic organs; and the _cardiac plexus_ behind the aortic arch for the thoracic viscera. Over these nerves one has no control. A blow in the region between the costal cartilages and below the sternum is a solar plexus blow and is very upsetting. The sympathetic system serves to maintain vitality in all the important portions of the system and one of its important functions is to keep up communication between one part and another, so that when any organ is affected the others will act accordingly and help out to the best of their ability. CHAPTER VI. THE BACK. [Illustration: FIG. 31.--The spinal column. (Church and Peterson.)]