the lungs of its victims. It is not difficult to
understand how these bacilli may be conveyed through the air from the
lungs of the sick to those of apparently healthy people. Such persons may,
however, be predisposed, either constitutionally or by defective hygienic
surroundings, to fall victims to this dreaded disease. Overcrowding, poor
ventilation, and dampness all tend to increase the risk of pulmonary
infection.

It must not be supposed that the tubercle bacillus is necessarily
transmitted directly through the air from the lungs of the sick to be
implanted in the lungs of the healthy. The germs may remain for a time in
the dust turn and _debris_ of damp, filthy, and overcrowded houses. In
this congenial soil they retain their vitality for a long time, and
possibly may take on more virulent infective properties than they
possessed when expelled from the diseased lungs.[37]

[Illustration: Fig. 94. Example of a Micro-Organism--Bacillus Tuberculosis
in Spotum. (Magnified about 500 diameters.)]

221. Ventilation. The question of a practicable and economical system
of ventilation for our homes, schoolrooms, workshops, and public
places presents many difficult and perplexing problems. It is perhaps due
to the complex nature of the subject, that ventilation, as an ordinary
condition of daily health, has been so much neglected. The matter is
practically ignored in building ordinary houses. The continuous renewal of
air receives little if any consideration, compared with the provision made
to furnish our homes with heat, light, and water. When the windows are
closed we usually depend for ventilation upon mere chance,--on the
chimney, the fireplace, and the crevices of doors and windows. The proper
ventilation of a house and its surroundings should form as prominent a
consideration in the plans of builders and architects as do the grading of
the land, the size of the rooms, and the cost of heating.

The object of ventilation is twofold: First, to provide for the removal
of the impure air; second, for a supply of pure air. This must
include a plan to provide fresh air in such a manner that there shall be
no draughts or exposure of the occupants of the rooms to undue
temperature. Hence, what at first might seem an easy thing to do, is, in
fact, one of the most difficult of sanitary problems.

222. Conditions of Efficient Ventilation. To secure proper
ventilation certain conditions must be observed. The pure air introduced