‘ _ PRESIDENTIAL ADDRESS. 637 
Tf hard work has to be done it is done under conditions of strain. The number 
of pathogenic organisms is increased in such places, and these two conditions run 
together—diminished immunity and increased mass influence of infecting 
bacteria. 
The volume of blood passing through, and of water vapour evaporated from, 
the respiratory mucous membrane must have a great influence on the mechanisms 
which protect this tract from bacterial infection. While too wet an atmosphere 
lessens evaporation, a hot dry atmosphere dries up the mucous membrane. 
As the immunising powers depend on the passage of blood plasma into the tissue 
spaces, it is clear that a proper degree of moisture is important. The temperature, 
too, must have a great influence on the scavenger activity of the ciliated 
epithelium and leucocytes in the mucous membrane of the nose. 
In the warm moist atmosphere of a crowded place the infection from spray, 
sneezed, coughed, or spoken out, is enormous. On passing out from such an 
atmosphere into cold moist air the respiratory mucous membrane of the nose is 
suddenly chilled, the blood-vessels constricted, and the defensive mechanism 
of cilia and leucocyte checked. Hence the prevalence of colds in the winter. 
In the summer the infection is far less. We are far more exposed to moving air, 
and the sudden transition from a warm to a cold atmosphere does not occur. We 
believe that infection is largely determined by (1) the mass influence of the in- 
fecting agent; (2) the shallow breathing and diminished evaporation from, and 
flow of tissue lymph through, the respiratory tract, in warm, moist confined air. 
Colds are not caught by exposure to cold per se, as is shown by the experience 
of Arctic explorers, sailors, shipwrecked passengers, &c. 
We have very great inherent powers of withstanding exposure to cold. The 
bodily mechanisms become trained and set to maintain the body heat by habitual 
exposure to open-air life. The risk lies in overheating our dwellings and over- 
clothing our bodies, so that the mechanisms engaged in resisting infection become 
enfeebled, and no longer able to meet the sudden transition from the warm atmo- 
sphere of our rooms to the chill outside air of winter. The dark and gloomy 
days of winter confine us within doors, and, by reducing our activity and 
exposure to open air, depress the metabolism; the influence of smoke and fog, 
gloom of house and streets, cavernous places of business and dark dwellings, 
intensify the depression. The immunity to a cold after an infection lasts but a 
short while, and when children return, after the summer holidays, to school and 
damp chill autumn days, infection runs around. The history of hospital gangrene 
and its abolition by the aseptic methods of Lister—likewise the history of insect- 
borne disease—show the great importance of cleanliness in crowded and much 
occupied rooms. The essentials required of any good system of ventilation are 
then (1) movement, coolness, proper degree of relative moisture of the air; (2) re- 
duction of the mass influence of pathogenic bacteria. The chemical purity of 
the air is of very minor importance, and will be adequately insured by attend- 
ance to the essentials. 
As the prevention of spray (saliva) infection by ventilation is impossible in 
crowded places, it behoves us to maintain our immunity at a high level. We may 
seek to diminish the spray output of those infected with colds by teaching them 
to cough, sneeze, and talk with a handkerchief held in front of the mouth, or to 
stay at home until the acute stage is past. 
In all these matters nurture is of the greatest importance, as well as Nature. 
A man is born with physical and mental capacities small or great, with inherited 
characteristics, with more or less immunity to certain diseases, with a tendency 
to longevity of life or the opposite, but his comfort and happiness in life, the 
small or full development of his physical and mental capacities, his immunity and 
his loxigevity of life, are undoubtedly determined to a vast extent by nurture. 
By nurture—using the word in its widest sense to include all the defensive 
methods of sanitary science—plague, yellow fever, malaria, sleeping-sickness, 
cholera, hospital gangrene, &c., can be prevented by eliminating the infecting 
cause; smallpox and typhoid by this means, and also by vaccination; and most of 
the other ills which flesh is supposed to be heir to can be kept from troubling by 
approximating to the rules of life which a wild animal has to follow in the matter 
of a simple, and often spare diet, hard exercise, and exposure to the open air. 
There is nothing more fallacious than the supposition commonly held that 
