6. The _Digestion_ and assimilation of food are aided by exercise, especially when taken in the open air. 7. _The nervous system_ is improved in nutrition and power by a moderate amount of exercise. In fact, a certain amount of muscular exercise is essential for a healthy mind. 8. _The elimination of urea_ is not increased by exercise. Evidently then it is not the metabolism of the nitrogenous substance of the muscles which supplies the energy for muscular contraction; but of the other oxidisable and non-nitrogenous substances (such as glycogen and sugar) contained in them. In practice it is found that with exercise more nitrogenous and non-nitrogenous food are both required. =Effects of Excessive Exercise.=—After prolonged exertion muscles become _exhausted_. This is associated with an accumulation in the muscles of the products of their action (especially sarcolactic acid). Then rest becomes necessary, in order that the effete products may be removed, and the nutrition of the muscles restored. Long-continued over-exertion produces _chronic exhaustion_, which may, if excessive, cause wasting of muscles. Exhaustion is much more liable to occur when a small group of muscles are exercised out of all proportion to others. Thus, in clerks, we have what is known as the _writer’s_ or _scrivener’s palsy_. The muscles of the hand, and especially of the thumb, cease to respond to the volition of the writer, but are seized with spasm every time writing is attempted; and the muscles of the thumb tend to waste. A similar condition sometimes arises in violinists, tailors, etc. The practical inference from these facts is, that one group of muscles should not be exercised disproportionately to the muscles of the rest of the body, and that proper intervals of rest should be allowed. Excessive exercise of the whole muscular system is very apt to harm those of previously sedentary habits. A walking tour entered on with more zeal than discretion, and not taken by easy stages for the first few days, is often productive of more harm than good. In the intervals of great mental labour, as with students, the amount of exercise should not be _suddenly_ increased, but should be regular and moderate in amount. Competitive exercise should be strictly regulated. The Oxford and Cambridge crews have been said to acquire heart-disease more commonly than the average, but this is not correct. Hypertrophy of the heart may occur as the result of severe exercise, and this within certain limits is not an abnormal condition. Occasionally dilatation of the heart has been produced in weakly lads.