4.  By means of continuous surfaces.  The bacteria may either grow along such surfaces forming a continuous or more or less broken layer, or may be carried from place to place in the fluids which bathe them.

All these modes of extension are well shown in tuberculosis.  This disease is caused by a small bacillus which does not produce spores, has no power of saphrophytic growth under natural conditions, and is easily destroyed.  Moisture and darkness are favorable conditions for its existence, sunlight and dryness the reverse.  There are three varieties or strains of the tubercle bacilli which infect respectively man, cattle and birds, and each class of animals shows considerable resistance to the varieties of the bacillus which are most infectious for the others.

The primary seat of the infection in man is generally in the upper part of the lung.  The organisms settle on the surface here and cause multiplication of the cells and an inflammatory exudate in a small area.  With the continuous growth of the bacilli in the focus, adjoining areas of the lung become affected, and there is further extension in the immediate vicinity by means of the lymphatics.  Small nodules are formed and larger areas by their coalescence.  Infection with tuberculosis is so common that at least three-fourths of all individuals over forty show evidences of it.  The examination of two hundred and twenty-five children of the average age of five years who had died of diphtheria showed tuberculous infection in one-fifth of the cases and the frequency of infection increases with age.  The defence on the part of the body is chiefly by the formation of dense masses of cicatricial tissue which walls off the affected area and in which the bacilli do not find favorable conditions for growth.  This mode of defence, which is probably combined with the production of substances antagonistic to the toxines produced by the bacilli, is so efficacious that in the great majority of cases no further extension of the process takes place.  In certain cases, however, the growth of the bacilli in the focus is unchecked, the tissue about them is killed and becomes converted into a soft semi-fluid material; further extension then takes place.  All parts of the enormous surface of the lungs are connected by means of the system of air tubes or bronchi, and the bacilli have favorable opportunity for distribution, which is facilitated by sudden movements of the air currents in the lung produced by coughing.  The defence of the body can still keep pace with the attack, and even in an advanced stage the infection can be checked in some cases permanently; in others the check is but temporary, the process of softening continues, and large cavities are produced by the destruction of the tissue.  On the inner surface of these cavities there may be a rapid growth of bacilli.