tive system, and especially of those affections which are of a parasitic nature. Animal and vegetable food which is charged with parasitic life is unclean food, and, as we have seen, is a cause of some of the most troublesome as well as of some of the most dangerous affections. Water containing organic or inorganic impurities is uncleanly, and is a cause of many diseases affecting the alimentary system. Thus water as the bearer of organic virus is, indirectly, the uncleanly cause of the disease called typhoid fever, and, as the bearer of the inorganic poison, called lead, it is the indirect cause of lead colic. Some affections of the nervous system are induced by uncleanliness in feeding. Thus those who work in lead, and do not take care to cleanse their hands, are, as we have seen, subjected to paralysis from absorption of lead. The same fact applies to uncleanliness in regard to other agents which are absorbed by the skin in uncleanly occupations and which affect the nervous centres.

Air rendered uncleanly is a cause of numerous diseases affecting the blood and the circulatory and respiratory systems. We have seen this fact illustrated in a variety of ways in the preceding pages. All the diseases produced by impure vapors and by dusts are due to this form of uncleanliness, as well as the affections attributable to a deteriorated or devitalized atmosphere.

The cutaneous or external membranous surface of the body is of all parts most subject to suffer from uncleanliness of person. Many of the parasitic diseases, scabies particularly, are determined by uncleanly habits though they be not directly generated. Sores upon the skin and abrasions in folds of the skin are the common results of accumulation of dust and dirt on the surface. We have seen how in disease from paraffin and soot (pp. 37072) the skin suffers in this manner. Uncleanliness of the skin leads also to general ill health of body. When the skin acts imperfectly from being oppressed with a covering which closes up the perspiratory ducts and prevents free evaporation of water from the body, there is an excess of transpiratory function, vicarious function, thrown upon the lungs. When there is excess of work, vicarious work, thrown upon the lungs, the liver soon begins to suffer, and the digestion becomes, thereupon, deranged. With this there is accumulation of gas or flatus in the stomach and intestines and greatly impaired digestive activity. Hence persons

who are not given to ablution of the body are always troubled with flatulency and indifferent digestion.

In addition to the above-named causes, I might add those which have passed before us as worry, unnatural physical exertion, and moral contagion, all causes which tell, primarily, upon the nervous systems, and afterwards upon those organs which depend upon the nervous centres for their vital power.

CHAPTER XI.

SENILE DEGENERATIVE CAUSES OF DISEASE.

To all these various classes of disease, divided, so far, in respect to origin, into six groups, must be added those changes which are specially incident to old age, and which are said to indicate the progress of senile decrepitude and decay.

Each period of life is marked out as belonging to one or other of four distinct stages of progress. There is a first period extending from infancy to complete adolescence, through a range say of thirty years. There is a second period extending from thirty to forty-five years, which embraces a time that reaches from completed adolescence up to completed manhood. There is a third period extending from forty-five years to about sixty-five, which includes a time when the body has attained what may be called full lifehood, and retains it without much shade of change if the conditions necessary for natural life are fairly observed. Lastly, there is a period prolonged from sixty-five to ninety or even a hundred years, in which the body is passing from completed lifehood into natural decline.

The first two of these stages seem to be fixed and ordained by strict rules amongst members of the human family. The two last-named periods are not so fixed, but either of them may be reduced or extended. Some men and women have passed into their declining stage so soon as they have reached perfected lifehood, and die without showing any third or retaining stage at all. Others retain fair perfection of lifehood even up to the seventieth year, and decline so slowly that their fourth stage may be extended many years beyond the ordinary duration: these seem, positively, to live a new life, to experience a rejuvenescence under which they may attain to even a hundred years.

Through all these stages there is a certain line of true physical change. In the first stage all the organs and parts undergo

the enlargement and development which constitutes what is called growth. In this stage the elastic tissue of the elastic mobile organs is brought up to its full range of tension, to as much as it can bear with equality of resistance and with evenness of action. In the second stage the elasticity is maintained, but is not imposed upon by further force of growth, while all the organs, brought now to their fulness of development, are consolidated and fashioned into working order. In the third stage the perfected organism, though no longer naturally expanding or growing, and no longer possessed of its previous elasticity, is enabled, nevertheless, to endure a certain long and defined phase of work and movement. It does not truly repair so readily as it did; does not bear shocks, mechanical or mental, so well as it did; but still it works on, a good steady, hardy organism, and in some powers of a mental kind is better endowed than at any previous period of its existence. In the last stage the physical decadence of the organism is the notable fact. In this stage the once elastic fabric has ceased to be elastic, and affords little resistance or resiliency. The once elastic arterial tubes which recoiled after each stroke of the heart, and by counter-stroke helped on the circulation of the blood, gradually fail. The resilient lungs which, by their elastic fibres, responded to the impulse of the respiratory muscles, gradually fail, so that the emptying of the lungs of their contained air is imperfect, and breathing is shortened. Then within the tissues of the relaxed organs new products ceasing, under an impaired nutrition, to be formed, the organs shrink. Then the blood, circulating more languidly, and diminished, is reduced in volume and in vital warmth. Then the brain and nervous structures undergo the physical change called sclerosis. Then, at last, all organs and parts passing gently into inertia, the extinction of life from the periphery or circumference of the body towards its centres, leads to complete arrest of motion, or, as it is said, death.

It is in this last stage that the phenomena of the natural disease by which life is terminated is developed. The disease is sometimes located in one organ, sometimes in another, and by the organ most affected the mode of death is commonly recorded. Really, however, when at the full period of life one vital organ fails, so dependent are all the vital organs on each other, they all soon begin to fail with their fellow, and follow it into its deathly decrepitude. From observation founded on this fact came the

wise saying of one of the shrewdest of philosophers, that old men are like ruined towers." They hold up, wonderfully, so long as they hold up together and as one; but touched at a single part they fall in mass.

Because their vital activity is less than it was in earlier times of life, old persons are less liable to be affected by some of the acute diseases that are incident to the early days of vital power. Thus they are saved from various causes of danger. For the same reason, however, they are more exposed to danger when they are attacked with maladies of an acute character, and sink easily from even slight attacks of acute disease.

The period of the year in which the aged most frequently succumb extends from the close of November to the end of April. The period of highest mortality or absolute maximum is January. These are the periods marked by cold and dryness and cold.

The period when the mortality from old age or senile decay is the least extends from May to November; the time of lowest mortality or absolute minimum being from July to October.

So soon as the month of November is on the advance mortality from senile decay begins rapidly to rise, an indication that the wave of cold, now setting in, is telling upon the reduced vital powers.

The organs of the body which are most frequently points of failure are the lungs, a fact which has led some authorities to conceive that in every case of natural decline the primary failure is from the respiratory surface. A dogmatic statement to such effect is, I think, too absolute. But it must be conceded that as no surface of the body is so directly affected by cold air as the breathing surface of the lungs, this is the surface which in a large majority of cases is primarily affected. Hence the number of aged people who, during the season of cold, and of cold and damp, sink from bronchitis, acute or chronic, pulmonary congestion, asthma, spasmodic breathing, and other affections of the chest.

It often happens owing to this weakness of respiration that aged people going from a warm room, in winter time, to sleep in a bedroom where a fire is not retained during the whole of the night, are subjected to acute congestion of the lungs as the result of the rapid fall of the temperature of the air they breathe. I have known in my experience several instances in which this sudden change has been sufficient to cause dissolution during the act of sleep.