The notable increase in the frequency of nervous diseases in modem times gives to this class of affections on importance far greater than has been attached to them at any previous period in the history of the race. A century ago, the literature upon the subject of nervous diseases was comparatively a meager one; bvt at the present time, there is probably no class of affections which commands a larger share of the attention of medical authors than this. Ziemssen's encylopedia of medicine has four ponderous volumes devoted to the subject, and voluminous works on single diseases, or classes of diseases, of the nervous system are becoming very numerous.

The cause of the great increase of attention given to this subject is, without doubt, the great increase in the number and frequency of nervous disorders. The increasing tendency in this direction is a subject of marked concern on the part of many observing and reflecting physicians; and it is a matter of importance to consider briefly, at least, some of the causes which have led to this remarkable development of a special class of diseases in recent times.