DISEASE. •289 DISEASE. suit from the prcseiiw, growth, and multiplica- tion of living organisms. Following the publica- tion and discussion of these statements arose the theory of spontaneous generation of minute animal and vegetable organisms, advocated by Needhani, in 1749, after looselj- conducted ex- periments. Bonnet, in 17G2, Lazarus and Spal- lanziini in 1709, Schultze in 1830, .Schwann in 1S37, Helmholtz in 1S43. Schroeder and Von Dusch in 1854. Hoffmann in 1800, Chevrcul and Pasteur in IStil. all contributed results of ac- curate scienlitie experiments which showed the absolute falsity of the theory of spontaneous generation. But. incredible as it may now seem, it was not till 1870 that the combined results, philosophical, chemical, and biological, of Tyn- dall and Cohn set the question at rest and estab- lished the fact that '"all life comes from life," to use the words of Harvey's law. About 1873 the germ theory of disease was authoritatively restated, and germ causation became an accepted principle. Gekms IX THE BoDT. The question as to how germs enter the body and the means of defense against them which the body possesses is one of extreme importance. When we think of the om- nipresence of germs, in the air we breathe, in the water we drink, in much of the food we eat, in the soil upon which we walk, in fact in or upon almost everything we touch or with which we come in contact, the question why bacteria <lo not oftener enter the body tissues would seem to be more rational than why they sometimes do so enter. Fortunately most germs are harm- less; but many are capable of producing the most severe forms of disease. There are con- stantly present in the mouth, nose, and upper air- passages, as well as throughout the entire gastro- intestinal canal, micro-organisms which, under ordinary conditions, are perfectly harmless. Then, too, the body is so built as to offer very powerful resistance to the entrance into it of most germs. First in importance of the body defenses against germ invasion is the skin. The unbroken skin offers an almost impassable bar- rier to the passage of most forms of germs. Being the most exposed, it is also the strongest of the bod.v defenses against the entrance into it of micro-organisms. Few. if any, germs have the power to penetrate it, if healthy and intact. Any person accustomed to dissecting, operating, or performing autopsies with ungloved hands, not- ing with what impunity highly infectious ma- terial may be handled provided there is no broken surface, and how serious are often the results of an overlooked wound or a chance scratch, realizes the importance of the skin as a protec- tion against germs. There are, however, certain normal openings in the skin. These are: (a) those of the digestive tract — the mouth and anus, communicating with the oesophagus, stom- ach, intestines, and rectum; (b) those of the respiratory tract, the mouth and nose — leading into the larynx, trachea, bronchi, and lungs; (e) those of the senito-urinary tract, the urethra and vagina — leading to the bladder, ureter, kidney, uterus, and Fallopian tubes; (d) the eye. These openings and tracts are lined by mucous mem- branes which may be considered as forming a second line of defense against germ invasion. Beinff less exposed than the skin, the mucous membranes are also less resistant than the skin to the entrance of semis. Indeed, to certain species of germs the mucous membranes are especially susceptible ; as the mucous membrane of the intestine to the bacillus of typhoid fever, that of the respiratory tract to the diplitheria bacillus and to the bacillus of pneunmnia, and that of the genito-urinary tract to the gonococcus. To a great many of the most dangerous species of germs, however, the healthy nuicous membrane, like the unbroken skin, presents an impassable line of defense. Xor is the body helpless against the activities of germs even after they have entered its tissues. While the lymphatic system often furnishes the channels by means of which germs are carried from one part of the body to another, the lymphatic glands themselves un- doubtedly act as lilters arresting the progress of the germs, while the lymph-cells probably pos- sess certain germ-destroying powers. Then, too, the white corpuscles of the blood are endowed with properties which enal)le them to take up germs and destroy them. It is largely for this jiurpose, iu all probability, that the white blood-corpuscles leave the vessels and pass out into the surrounding tissues in inflammatory, conditions. Certain other cells probably possess this same power of destroying germs. Such cells are known as phagocytes, and the process of germ destruction by them is called phagocytosis. There are also developed in the body during the prog- ress of germ infections certain substances which appear to be in solution in the fluids of the blood. These su^jstances are known as antitoxins (q.v.), and have a distinctly inhibitory etl'ect on the further development of the germs. The effects upon the body mechanism of the activities of disease-producing genns vary, of course, greatly for the different germs. Certain general modes of action ma,v, however, be men- tioned. (1) The direct (usually local) effect of the presence of the germs. Thus, in diphtheria the direct local effect of the germ in the throat is death of parts of the mucous membrane and the fonnation of what is known as a false mem- brane. In a similar manner the ulcers. in the intestines in typhoid fever are due to the direct local action of the typhoid bacillus. Another local effect which germs may sometimes have consists in the formation of infectious emboli iu different parts of the body. Thus, for in- stance, in infectious endocarditis with bacterial growth in the heart valves, a little mass of bacteria may become detached, and carried through the circulation until it reaches a vessel too small for it to pass through. Here it stops, forming an infectious embolus and thus setting up a new focus of infection. (2) The produc- tion of toxins (q.v.). These are poisons pro- duced in the bod.v by the activities of bacteria. They seam to be largely present in the plasma of the' blood and are consequently distributed throughout the body. It is to these toxins that the .systemic symptoms of an infectious disease are due, e.g. the fever, prostration, delirium, etc. These toxins differ for different bacteria, a par- ticular toxin probably being specific for each species of germs. That a disease may be ac- cepted as positivelv proved to be of germ origin, it must fulfill certain veiy rigid conditions. One single species of germ must always be found present in the diseases. This germ must not be found regularly in connection with any other disease. It must be possible to cultivate this germ artificially, and to separate it in what is
