Ganglion (Gr. a little swelling), in anatomy, a small rounded or elongated nervous mass, of a reddish gray color, situated in the course of the nerves. There are two kinds of nervous ganglia, one forming part of the cranial system of nerves, the other part of the sympathetic system; the first kind are situated near the origins of many of the cranial and of all the vertebral nerves, and on the posterior or sensory root of the latter; the second are generally placed along the sides of the anterior surface of the spinal column, from the head to the coccyx, the two great semilunar and cardiac ganglia coming near the median line. They are composed of two substances, one white like the medullary substance of the brain, the other reddish gray, somewhat resembling the cerebral cortical substance; the internal medullary filaments are the continuation of the nerve upon which the ganglion is situated. The sympathetic system of ganglia is considered by some as a series of more or less independent centres, giving off nerves to the organs of nutrition or communicating branches to the cerebro-spinal system; according to others, these ganglia and their associated nerves form a special system with numerous ramifications, the sympathetic system presiding over the involuntary contractions of the heart and digestive apparatus, and all the processes concerned in secretion, nutrition, and exhalation, and in disease conveying different sympathetic phenomena from one part of the organism to another.

Strictly speaking, all the nervous centres in the highest vertebrates may be called ganglia; even the hemispheres of the human brain may properly be styled cerebral ganglia. The principal ganglia of the head are the ophthalmic, which sends branches to the iris and the vascular apparatus of the eyeball; the otic, intimately connected with the organ of hearing; Meckel's or the spheno-palatine, ministering to the senses of smell and taste; the submaxillary, whose branches proceed almost entirely to the gland of that name; the Gasserian, of the fifth pair of nerves; and those near the roots of the pneumogastric and glossopharyngeal. In the neck are the superior, middle, and inferior ganglia of the sympathetic, and the origins of the cardiac plexus which supplies the heart; in the chest, the 12 thoracic ganglia on each side, from which originate the splanchnic nerves which go to join the semilunar ganglia and the solar plexus; in the abdomen, the latter sends branches which accompany all the divisions of the aorta. There are besides these the lumbar and sacral ganglia on each side of the spine, distributing their branches to the organs in the pelvis. The so-called lymphatic ganglia are glandular, and not nervous masses.

In the invertebrata ganglia are the highest form of nervous centres, and occur either isolated or connected together by single or double longitudinal cords; they perform the functions both of the cerebral and spinal centres of the higher animals.-In surgery, a ganglion is a small indolent fluctuating tumor, developed in the course of the tendons, containing a semi-fluid secretion enclosed in a cyst, generally communicating with the tendinous sheath. It is a dropsy of the synovial sheath, caused by friction, some wrench or tension of the tendon, or the sequence of some rheumatic or gouty disease; the light of a candle may be seen, through it. The most common situation is about the wrist and fingers, though it may occur in the course of any ten-don. When there is no inflammation, the best treatment is to puncture the tumor by the subcutaneous method, in order that the contained fluid may escape into the surrounding areolar tissue and be absorbed; pressure and cold applications should then be applied. If this fail, stimulating liniments and even blisters may be tried, to induce absorption.

When unconnected with a tendinous sheath, the tumor may be dissected out, punctured like an abscess, or transfixed with a seton; the subsequent thickness may be removed by the vapor or steam bath and douche; any rheumatic taint requires to be corrected by appropriate remedies. A popular way of treating these tumors is to rupture them by a strong and sudden blow; those on the back of the wrist and hand may thus be scattered without danger.