ANATOMICAL, AND t»HYSIOLOGiCAL FACTORS 77 



organisms are showered throughout various tissues. In endo- 

 carditis and also in cases of thrombosis associated with infection, 

 mycotic emboli are occasionally broken off and carried by the blood 

 stream to other parts of the body. The new foci of infection are 

 called metastatic foci. 



Direct Extension. — This method of spread of infection is seen 

 in gas gangrene where the process extends from one muscle cell to 

 another. While in abscess formation the initial infection is fre- 

 quently of metastatic origin yet the enlargement of the abscess is 

 due largely to direct extension. 



Because fascia and tendon sheaths act as barriers to direct exten- 

 sion through them, abscesses take the course of least resistance and 

 spread along fascial planes and tendon sheaths. Direct extension 

 of a retropharyngeal abscess along the prevertebral fascia and of 

 the spread of infection by direct extension in the submaxillary sub- 

 cutaneous tissue is illustrated in Fig. 1. 



Factors Involved in Pulmonary Infection. — It is obvious that 

 infection of the lungs may result from aspiration of contaminated 

 food material such as raw milk down the trachea, by extension 

 along the surface or within the walls of the trachea, through the 

 lymphatics from the cervical region, through the blood stream, 

 from contact of the pleura with an infected pericardium or dia- 

 phragm, by direct trauma such as occurs in chest injuries or by 

 direct extension of infection from the chest wall as might occur in 

 an infected malignancy of the breast that had eroded into the 

 pleural cavity. Small abscesses of the lung, located at the surface, 

 may rupture into the pleural cavity and give rise to empyema or 

 pus in the pleural cavity. It should be remembered that destruc- 

 tion of surface barriers by malignant growths offers excellent por- 

 tals of entry for infection. 



Lobar Pneumonia. — Hadfield and Garrod (1938) give an excel- 

 lent summary of important research bearing upon lobar pneumonia. 

 They state that Blake and Cecil (1920) definitely settled the route 

 of infection as being via the respiratory tract and not by hematog- 

 enous or lymphogenous routes. Their results and those of Still- 

 man (1930) make it seem improbable that a downward spread on 

 the mucous membrane occurs but that instead the organisms may 

 be carried in by small inhaled droplets which penetrate the lung 

 deeply before coming to pest during inspiration. Hadfield and 



