No one can be long healthy, if deprived of fresh air and sufficient exercise; the term sufficient, however, must be regulated by the health, habits and nature of the individual. Some persons have been accustomed from early youth to walk several miles a day, or to take other active exercise, on horseback, in rowing, cricket, gardening, etc. Such persons, of course, require much more exercise to keep them in health than those of a more delicate constitution, or those whose occupations have been of a more sedentary character. To those who are fond of it, horse exercise is decidedly the best; walking, rowing and cricket are all good; swimming, if not continued too long at a time, is good in summer for those whose constitutions are strong enough to admit of it. Skating cannot be recommended on the score of health. If people could be satisfied with skating for an hour or so at a time, no mischief would result from it; but when young and delicate girls, not satisfied with skating several hours in the day, must also skate for two or three or four hours at night, with the thermometer perhaps near zero, who can wonder at the frequent serious result. So with dancing; a girl, lightly clad, will dance through a cold winter's night, and then, return home, a little before daybreak, perhaps several miles. It is well known that three-fourths of the cases of consumption in this country have their origin in these amusements.

Excessive bodily exertion of various kinds is a common exciting cause of disease. The heart, excited to inordinate action, is often strained and distended, and its function, or even its structure, and that of the great vessels, may be impaired in consequence. This is especially apt to happen if there be anything already imperfect in the structure of the organ, its valves or vessels; and there are naturally very various degrees of perfection and strength in these parts.

The brain is particularly liable to suffer from violent exertion, especially if joined with a stooping or constrained posture; for its vessels are not, like those of the limbs and trunk, supported by muscular pressure upon them, and the excited heart can therefore send its blood into them with more force. Hence giddiness, noise in the ears, deafness, defective vision, convulsions, palsy, apoplexy, have been brought on by violent exertion.

The lungs are also apt to suffer; for the blood being returned to them faster than they can arterialize it, they become greatly congested; hence cough, shortness of breath, bleeding at the lungs, or inflammation of the lungs may ensue.

Other internal organs are sometimes disordered by the blood thrown or retained in their vessels by the pressure of external muscular action. Derangement of the liver, vomiting of blood, piles, etc, have been brought on by such a cause. The sharp pains or stitches felt in the sides or abdomen, on running fast, are commonly supposed to be in the liver or spleen; but more probably they are spasms of the intestines-temporary colic, produced by irregular pressure on them when overcharged with blood.

Some kinds of muscular exertion peculiarly effect certain organs. Thus, loud reading or speaking, or blowing wind instruments, especially tries the organs. Excessive or rough riding or leaping may injuriously affect the kidneys and organs of generation. Straining to lift a heavy weight, or at stool, or in any continued effort which implies holding the breath, endangers the structure of the vessels of the chest and brain.

Bodily exertion, long continued, may also cause disease by its exhausting effects. In extreme degrees this exhaustion may amount to fainting, and even death; short of this it may cause great weakness of muscles, and of the heart, with corresponding depression of other functions; hence arises the low typhoid fever which sometimes follows prolonged fatigue. In slighter cases, we have giddiness, faintness, nausea, loss of appetite, indigestion, costiveness, and other varieties of injured function. Excessive fatigue may cause such an amount of sleeplessness as to bring the patient into a state almost resembling delirium tremens.

Strong mental emotion is a common cause of disease. "Closely knit together as the mind and body are, it is not surprising that they should be ever ready to affect each other. The heart most remarkably suffers from such causes. Thus, a sudden shock, whether of grief, surprise, fear, or even joy, may cause fainting, nay even death itself has ensued; and the expressions "frightened to death," and "killed with joy," are not always mere figures of speech. Sudden acute pain often causes fainting." Apoplexy, palsy, inflammation of the brain, epilepsy, and insanity, have been caused by excessive anger, terror, surprise and joy. A piece of very bad news frequently takes away appetite, or impairs digestion. Fright or anxiety often loosens the bowels, or brings on a bilious attack or jaundice.

Excessive evacuation, or loss either of blood or of some secretion, is frequently a cause of debility, which predisposes to other diseases; but if the loss be great or sudden, it may produce immediate disease. A certain fulness of the heart and blood-vessels is required for their healthy functions, as well as for those of all the organs which they supply. If a moderate quantity of blood be suddenly withdrawn, or a large quantity less suddenly, the heart's action will be impaired, rendered irregular, and may be interrupted, and the brain not receiving a current sufficient to maintain its functions, there may be fainting with loss of consciousness, accompanied or followed by disordered function, palpitation, delirium, convulsions, or by death. Lower mentions a case of extensive enlargement of the veins of the lower extremities, in which the patient could not stand without fainting until they were bandaged.

Inattention to the calls of nature is a prolific source of disease; when the bowels are not emptied as frequently as they should be, the faeces accumulate in them, become offensive, and the more fluid portions become absorbed into the system and poison the blood; frequently the coats of the bowels become ulcerated and diseased, and piles are a very common result of allowing the bowels to get and to remain costive. Retention of urine has even a more serious result. Loss of power, or even rupture of the bladder has not unfrequently occurred. Sometimes the neglect to empty the bladder when nature gives the hint, will cause an accumulation in that organ, over distention, followed by loss of power. The retained urine is liable to be partially re-absorbed into the system, giving a urinous smell to the breath and perspiration, and sometimes causing typhoid symptoms. The retained urine is also liable to decomposition; highly irritating and offensive matters are produced, which cause injury to the bladder, rapidly extending up the ureters to the kidneys. Checked perspiration is a well-recognized cause of disease, resulting in colds, coughs, rheumatism, inflammations and fevers.

An artificial or diseased discharge, or secretion as that of a seton or issue, or from an ulcer or diseased membrane, or an unnaturally profuse flow of an ordinary secretion-such as looseness of the bowels, if so long established as to become habitual, cannot be suddenly suppressed without great risk of exciting disease. Apoplexy has not unfrequently resulted from the sudden drying up of an old sore.

There can bo no genuine health without Cleanliness; an old pro-vcrl> says, that "cleanliness is next to godliness;" and certainly a want of cleanliness is a prolific source of skin diseases, fevers, and affections of all those organs that sympathize with the skin.