The network of sympathies that was fixed only on unsystematized resemblances, empirical observations, or a conjectural assignation of the nervous net work now rests on a strict analogy of structure: when the envelopes of the brain are inflamed, the sensitivity of the eyes and ears is sharpened; in the operation of hydrocele by injection, the irritation of the vaginal wall causes pains in the lumbar region; an inflammation of the intestinal pleura may, by a 'sympathy of tonicity', cause a cerebral affection.'