I have been careful here to state that combined overwork of mind and body is doubly mischievous, because nothing is now more sure in hygienic science than that a proper alternation of physical and mental labor is best fitted to insure a lifetime of wholesome and vigorous intellectual exertion.  This is probably due to several causes, but principally to the fact that during active exertion of the body the brain cannot be employed intensely, and therefore has secured to it a state of repose which even sleep is not always competent to supply.  There is a Turkish proverb which occurs to me here, like most proverbs, more or less true:  “Dreaming goes afoot, but who can think on horseback?” Perhaps, too, there is concerned a physiological law, which, though somewhat mysterious, I may again have to summon to my aid in the way of explanation.  It is known as the law of Treviranus, its discoverer, and may thus be briefly stated:  Each organ is to every other as an excreting organ.  In other words, to insure perfect health, every tissue, bone, nerve, tendon, or muscle should take from the blood certain materials and return to it certain others.  To do this every organ must or ought to have its period of activity and of rest, so as to keep the vital fluid in a proper state to nourish every other part.  This process in perfect health is a system of mutual assurance, and is probably essential to a condition of entire vigor of both mind and body.

It has long been believed that maladies of the nervous system are increasing rapidly in the more crowded portions of the United States; but I am not aware that any one has studied the death-records to make sure of the accuracy of this opinion.  There can be no doubt, I think, that the palsy of children becomes more frequent in cities just in proportion to their growth in population.  I mention it here because, as it is a disease which does not kill but only cripples, it has no place in the mortuary tables.  Neuralgia is another malady which has no record there, but is, I suspect, increasing at a rapid rate wherever our people are crowded together in towns.  Perhaps no other form of sickness is so sure an indication of the development of the nervous temperament, or that condition in which there are both feebleness and irritability of the nervous system.  But the most unquestionable proof of the increase of nervous disease is to be looked for in the death statistics of cities.

There, if anywhere, we shall find evidence of the fact, because there we find in exaggerated shapes all the evils I have been defining.  The best mode of testing the matter is to take the statistics of some large city which has grown from a country town to a vast business hive within a very few years.  Chicago fulfils these conditions precisely.  In 1852 it numbered 49,407 souls.  At the close of 1868 it had reached to 252,054.  Within these years it has become the keenest and most wide-awake business centre in America.  I owe to the kindness