From the lungs the bacilli are carried by the lymphatics to the lymph nodes at the root of the lungs, in which a similar process takes place; this, on the whole, is favorable, because further extension by this route is for a time blocked.  The extension by means of surfaces continues, the abundant sputum which is formed in the lungs and which contains large numbers of bacilli, becomes the vehicle of transportation.  The windpipe and larynx may become infected, the back parts of each are more closely in contact with the sputum and are the parts most generally infected.  A large part of the sputum is swallowed and infection of the intestine takes place, the lesions taking the form of large ulcers.  From the intestinal ulcers there is further extension by means of the lymphatics, to the large lymph nodes in the back of the abdominal cavity (Fig. 8-25); the bacilli may also pass from the ulcers into the abdominal cavity and be distributed over the surface of the peritoneum resulting in tuberculous peritonitis.  When the disease has reached an advanced stage, bacilli in small numbers continually pass into the blood and are distributed by this over the body, producing small nodules in many places.  In rare cases distribution by the blood is the principal method of extension, and immense numbers of small foci of disease are produced, the form of disease being known as acute miliary tuberculosis.  Although the bacilli are distributed everywhere, certain organs, as the brain and muscles, are usually exempt, because in these the conditions are not favorable to further growth of the bacilli.  Tuberculosis, although frequently a very acute disease, is usually one of the best types of a chronic disease and may last for many years.  The chronic form is characterized by periods of slow or rapid advance when conditions arise in the body favorable for the growth of the bacilli, and periods when the disease is checked and quiescent, the defensive forces of the body having gained the upper hand.  Often the intervention of some other disease so weakens the defences of the body that the bacilli again find their opportunity.  Thus typhoid fever, scarlet fever and other diseases may be followed by a rapidly fatal advance of the tuberculosis, starting from some old and quiescent focus of the disease.

Tuberculosis is also one of the best examples of what is known as latent infection.  In this the infectious organisms enter the body and produce primary lesions in which the organisms persist but do not extend owing to their being enclosed in a dense and resistant tissue, or to the production of a local immunity to their action.  Dr. Head has recently examined the children of households in which there was open tuberculosis in some member of the household.  By open tuberculosis is understood a case from which bacilli are being discharged.  He found with scarcely an exception that all the children in such families showed evidences of infection.  The detection of slight degrees of tuberculous infection