3 See A Systematic Treatise on the Principal Diseases of the Interior Valley of North America, etc., etc., 2d Series, pp. 795, 796.
A study of these same military statistics, representing the mean ratio of the prevalence of diseases of the respiratory passages for a period of ten years at nearly all the posts, will justify some other inferences of interest besides the one just stated. According to this general inference or rule, which is assented to by all the authors within my reach, the three important factors in the climates most favorable for producing bronchial inflammation are cold, variableness, and dampness, the latter being emphasized by most writers as of predominating influence. Yet the tables before us show that the highest ratio of prevalence of inflammatory attacks of the mucous membrane of the respiratory passages in the northern part of the interior valley was at Fort Snelling, in the immediate vicinity of St. Paul, Minnesota, being 600 attacks for every 1000 soldiers, while the lowest ratio was at Fort Dearborn, on the site now occupied by the city of Chicago, being only 102 for every 1000 soldiers. Looking at the posts in the eastern part of the northern belt of country, Madison Barracks, at Sackett's Harbor, at the eastern end of Lake Ontario, gives a ratio of 637 attacks for every 1000 soldiers, while Fort Niagara, at the mouth of the Niagara River, near the western end of the same lake, gives a ratio of only 355. Again turning to the posts in the southern belt of country, the tables show at Fort Jessup, in the interior of Western Louisiana, a ratio of 432.8, while at Fort Jackson the ratio was only 47.5 and at Fort King 101.2. As Fort Snelling is on the high rolling prairie of the interior of Minnesota, noted for its cold and dry air, and Fort Jessup on the elevated arid plateau between the head-waters of the Sabine and the Red River, they cannot be noted for a high degree of atmospheric moisture. On the other hand, Fort Dearborn was located on the south-west shore of Lake Michigan, on the borders of a low and wet prairie with a substratum of impervious clay, giving all the conditions favorable for the prevalence of a high degree of atmospheric moisture. And Forts Jackson and King are both on low alluvial lands only fifty miles from the Gulf. Again, Fort Niagara is surrounded by all the conditions favoring a high degree of atmospheric moisture, certainly equal to those surrounding Madison Barracks in nearly the same latitude, and yet the ratio of attacks in the latter was nearly double those in the former. It is evident, therefore, that there exists some important factor in the climatic relations of the inflammatory affections of the respiratory passages besides temperature, humidity, and changeableness. A glance at the topography of the whole country will show that each of the posts giving a high ratio of attacks—namely, Madison Barracks and Forts Snelling and Jessup, to which may be added Forts Gratiot, Crawford, and Wood—are so located as to be exposed to the prevalence of unusually severe winds or atmospheric currents either from the north-east or the north-west and west, with certain relations either to high mountain-ranges or ocean-currents. For instance, from Madison Barracks the open valley of the St. Lawrence River extends in a north-easterly direction to the Atlantic Ocean, where the cold ocean-current is from the north, favoring the pressure of cold atmospheric currents directly up the valley from the north-east, reaching its termination at the eastern end of Lake Ontario with but little diminution of force. The mountains of Northern New York, Vermont, and New Hampshire seem to prevent the deflection of these currents to the south, and help to keep them directly in the line of the valley. That the high ratio of attacks of bronchial and catarrhal affections at Madison Barracks is largely due to the influences here described is corroborated by the fact that the same class of diseases are much more prevalent in the province of Quebec, through which the valley of the St. Lawrence extends, than in the province of Ontario, as shown by the Registrar-General's Report in reference to the several military posts in the Canadas. Turning to Forts Snelling and Crawford at the north and Jessup at the south, we find them so situated in relation to the great mountain-chains to the west as to be fully exposed to the cold and strong atmospheric currents that sweep over the Plains from the north-west and west with such force as to justify the popular title of blizzards. Without consuming more time in details, it may be said that the force and direction of atmospheric currents have quite as much to do with the development of inflammations of the air-passages, including all grades of bronchitis, as either temperature or humidity.
As might be inferred from what has already been said in relation to the influence of climatic conditions, season of the year is also found to exert a marked influence over the prevalence of bronchial affections. Those parts of the year characterized by a low temperature, high winds, and frequent thermometric changes are accompanied by the highest ratio of prevalence of inflammations of the respiratory passages. Thus, the statistics compiled from the records of all the military posts by Drake show an average ratio for the four quarters of the calendar year of 119.8 for the first quarter, 72.7 for the second, 48.7 for the third, and 99.6 for the fourth.4 This corresponds closely with the results of clinical records kept under my own observation through a series of years.
4 See Drake on the Principal Diseases of the Interior Valley of North America, p. 792.
That tubercular deposits in the lungs, cancerous growths, emphysema, and previous attacks of bronchitis, all strongly predispose the patient to further attacks of the last-named disease, is proved by universal clinical experience.
EXCITING CAUSES.—Exposure to sudden and extreme changes in atmospheric temperature from warm to cold is almost universally regarded as the chief exciting cause of inflammation of the bronchial as of all other parts of the mucous membrane of the air-passages. More accurate and detailed observations, however, show that such changes of temperature are seldom productive of diseases of this class unless accompanied by coincident high winds and humidity. My own studies concerning the relations between special meteorological conditions and the prevalence of particular diseases have led me to the following conclusions in regard to bronchitis and inflammation of the mucous membrane of the air-passages generally:
First. Many sporadic cases are caused, at any and all seasons of the year, by exposure of limited portions of the cutaneous surface to cool or cold currents of air while the rest of the body is well protected. Females going out with thin shoes and stockings or sitting before open windows with low-necked dresses, and children out on cold days with naked legs from short trousers and defective stockings, afford many and familiar examples of bronchitis from this cause.
Second. The sudden transition from a protracted period of intense dry cold to a higher temperature with increased atmospheric humidity. Almost every winter season, in the northern belt of the United States, east of the Rocky Mountains, is characterized by several periods of steady dry, cold air, varying from one to three weeks in duration, during which the mercury in the thermometer often descends more than 20° C. (8–10° F.) below zero, and which generally ends in a sudden change in the direction of the winds and a marked elevation of temperature, constituting what is popularly called a thaw. Such changes are very uniformly accompanied by a general prevalence of catarrhal affections of the air-passages, including many cases of bronchitis. This class of cases occur principally in the months of December, January, and February.
Third. The occurrence of those cold north-east winds that during the latter part of autumn and early part of spring so often sweep over the whole extent of our Atlantic coast and press up the valley of the St. Lawrence to the great interior lakes, and the still more severe currents that come during the same seasons from the north-west and west, over all the wide plains that intervene between the great mountain-chains to the west and the upper lakes and Mississippi River to the east, are also accompanied by a high ratio of prevalence of bronchial affections, as has been already shown from the records of the several military posts. Most of these severe storms of wind are accompanied by either snow or rain and a marked increase of ozone or active oxidizers. In some of the severe snowstorms from the north-east, occurring in the latter part of February and in March, I have found an unusual amount of free ammonia. Whether either the ozone or the ammonia has had anything to do with the production of the bronchitis cannot be determined until the observations and records now being made under the auspices of the American Medical Association have been continued for a few years, by which adequate data will be furnished for reliable deduction.
Besides ordinary meteorological conditions, bronchitis may be produced by inhaling irritating substances, such as steam, irritating gases, steel-dust, or minute particles of other metals or stone in workshops, and the dust encountered in handling grain, etc. The disease has often occurred in epidemic form without the presence of an obvious exciting cause. It also frequently occurs in connection with certain general fevers, more particularly with typhoid, measles, influenza, and pertussis. It also sometimes, though more rarely, accompanies rheumatism, constitutional syphilis, and erysipelas. The presence of tuberculous and cancerous deposits in the lungs almost always provokes more or less bronchial inflammation during some part of their progress.