Tired man, your nerves are sick! 
    They need rest and a tonic to restore
      their worn-out depleted cells!

No wonder people have believed this kind of thing.  It has been dinned into their ears for many years.  They have read it with their breakfast coffee and gazed at it in the street cars and even heard it from their family physicians, until it has become part and parcel of their thinking; yet all the time the fundamental idea has been false, and now, at last, the theory is exploded.

So far as the modern laboratory can discover, the nerves of the most confirmed neurotic are perfectly healthy.  They are not starved, nor depleted, nor exhausted; the fat-sheath is not wanting, there is no inflammation, there is nothing lacking in the cell itself, and there is no accumulation of fatigue products.  Paradoxical as it may sound, there is nothing the matter with a nervous person’s nerves.  The faithful messengers have borne the blame for so long that their name has gotten itself woven into the very language as symbolic of disease.  When we speak of nervous prostration, neurasthenia, neuroses, nervousness, and “nerves” we mean that body and mind are behaving badly because of functional disorder.  These terms are good enough as figures of speech, so long as we are not fooled by them; but accepting them in their literal sense has been a costly procedure.

Thanks to the investigations of physiologist and psychologist, usually combined in the person of a physician, “nervousness” has been found to be not an organic disease but a functional one.  This is a very important distinction, for an organic disease implies impairment of the tissues of the organ, while a functional disorder means only a disturbance of its action.  In a purely nervous disorder there seems to be no trouble with what the nerves and organs are, but only with what they do; it is behavior and not tissue that is at fault.  Of course, in real life, things are seldom as clear-cut as they are in books, and so it happens that often there is a combination of organic and functional disease that is puzzling even to a skilled diagnostician.  The first essential is a diagnosis as to whether it be an organic disease, with accompanying nervous symptoms, or a functional disturbance complicated by some minor organic trouble.  If the main cause is organic, only physical means can cure it, but if the trouble is functional, no amount of medicine or surgery, diet or rest, will touch it; yet the symptoms are so similar and the dividing line is so elusive, that great skill is sometimes required to determine whether a given symptom points to a disturbance of physical tissue or only to behavior.