Abnormal external influences may cause anemia, if they exert their effects on the system for a protracted period, or continuously, and in this way inhibit the activity of the individual organs. Among the more impor tant of these conditions may be mentioned : insufficient nourishment ; dark, unhealthy, cold, and damp living-quarters ; the effects of extreme heat or cold either in the open air or in poorly ventilated work-rooms ; over-exertion, either bodily or mental ; lack of sufficient sleep or improper sleeping accom modations ; any occupation which necessitates a stooping posture, so that proper respiration is interfered with ; excessive or insufficient muscular activity ; exertion which includes but one side of the body ; and, finally, marked mental disturbances. Attention must also be directed to the harmful character of the corset, which not only interferes with the normal expansion of the thorax, and the entrance of the air to the lungs, so that the ready combination between the oxygen and the red blood-cells is pre vented, but also exerts pressure on the important digestive organs, the stomach and liver, and in this way obstructs the abdominal circulation and the necessary movements and expansion of these organs. Indigestion and constipation result from the, latter conditions and are accompanied by numerous painful sensations.
It is the function of the physician to determine \vhich of these numerous factors are the essential causes of arimmia, bearing in mind that season and other circumstances may largely influence these conditions. Thus, many persons are capable of doing considerable muscular work in the winter, which it would be impossible for them to accomplish in the summer ; while others, apparently, are harmed only by their exertions during the winter, although this may be due to the fact that they cannot get along without the proper amount of sunlight, or that an atmosphere which is very dry or very moist has an unfavourable influence on respiration and muscular activity. The change from one season to another has an important bearing on the production of anaemia and other blood-disorders, so that in some latitudes a summer and winter anaemia can be differentiated ; and persons are seen who suffer regularly either in one or the other of these seasons from symp toms of anaemia. This is due to the fact that the changes induced in the blood circulatory system by the passage of the seasons cannot be withstood on account of some constitutional weakness or especially unfavourable surroundings ; or that the influence of some characteristic attribute of the seasons—heat, cold, dampness, etc.—cannot readily be overcome. The transition must be gradual in order that the individual may become accustomed to the change ; and the production of severe catarrhal troubles with the advent of either spring or autumn is a matter of common obser vation.
Marked changes in the circulatory system and in the blood may be caused by a great variety of diseases, among which may be primarily men tioned those of the digestive tract, the infectious diseases, and inflamma tion of the kidneys. Both anaemia and chlorosis may develop in children after an attack 61 summer diarrhoea, measles, scarlet fever, malaria, diph theria, etc., or in adults after typhoid, although the disease may have appeared very mild and run a short course.
It is natural to suppose that the various chronic diseases of important organs, such as chronic pleurisy, pulmonary tuberculosis, cardiac diseases, Bright's disease, and long-continued suppuration, should lead to marked changes in the character of the blood.
In some cases intestinal parasites may he the cause of the anaemia, and patients should therefore not be definitely treated for the condition unless an examination of the stools in suspicious cases has been made. Many a case of anaemia with severe digestive disturbances has been cured by the administration of a vermifuge. During late years new forms of intestinal parasites have been imported and are now endemic. A condition now found sometimes among miners is the " hook worm " disease, or " tropical chlorosis," which is due to a round worm, the Anchylostonta duodenale. Pregnancy and labour also exert their influence on the state of the blood.
The surroundings of the patient must be taken into consideration when there is some doubt as to the cause of the disease ; for these may lead to the production of an anemia where a tendency is present in the individual, or, if only slight evidences are at hand, may bring about a more severe degree of the affliction. Even hygienic measures which may be of signal value under certain circumstances, may sometimes lead to a very harm ful result, because they have been falsely applied. For example, when apparently perfectly healthy children who are growing actively and using up all their energies in accomplishing this growth, show a certain degree of fatigue and lassitude, because the nourishment demanded by the muscles is greater than the system can supply at once, it is an error to subject these children to cold baths or to urge them to swim, with the idea of strengthen ing their muscular system. As a result of such treatment very severe degrees of anemia may develop, because the body is unable to meet the double demands necessitated by increased muscular growth and the effort to keep warm. See SWIMMING.