CHEMICAL CEPTORS 109 



sponse to perforation by an external agent, respond 

 actively to the invasion of pneumococci, since these are 

 ancient enemies. The peritoneum, which throughout 

 phylogeny has been exposed to infection through 

 the genital tract and through the intestinal walls, has 

 an efficient mechanism for overcoming infection, but 

 makes no response when it is cut or burned. 



On the other hand, the brain and the lining of the 

 heart which have been protected as securely against 

 pyogenic invasion as from physical impacts are help- 

 less in the face of infection, as the external areas 

 are helpless before the X-ray. Once lodged in either 

 the brain or the heart, infection works its way pain- 

 lessly and persistently, undeterred by a local resist- 

 ance. The same inability to cope with the infrequent 

 invader marks all deep-seated areas which have had 

 no open connection with the outside world. Thus 

 below the surface of the face, neck, chest and ab- 

 domen, we find that the power of resistance to infec- 

 tion diminishes as the depth of the tissue below the 

 surface increases. The great vascular trunks buried 

 deep in the chest and abdomen show little resist- 

 ance to infection, although curiously enough, when 

 they emerge to more exposed positions in the thigh 

 and leg, they react to infection by swelling, pain 

 and other symptoms of inflammation, as do other 

 tissues which have been frequently subjected to infec- 

 tion from open wounds. The mediastinum has prac- 

 tically no mechanism of defense against infection. 

 An abscess may progress unheralded in the space 

 between the diaphragm and the liver, in the retro- 

 peritoneal region, in the mediastinum, in the pericar- 



