CONTACT CEPTORS 87 



inner structures of the mouth, where contact with en- 

 vironment is confined chiefly to the intake of food, 

 have had scant opportunity to evolve any important 

 defense mechanisms beyond those guarding against the 

 ingest ion of improper food. 



The same loyalty to the respiratory function and 

 indifference to other menaces was manifested by the 

 larynx, trachea and esophagus, when these parts were 

 subjected to injury under anesthesia. Instant arrest 

 of the respiration was caused by even gentle contact 

 with the mucous membrane of the larynx at any point 

 from just below the vocal cords to the upper laryngeal 

 opening and the under surfaces of the epiglottis. 

 Injuring the trachea caused coughing, but injuring 

 the esophagus caused no response. Extensive dis- 

 sections of other tissues of the neck caused no disturb- 

 ance of the blood-pressure and respiration provided that 

 the vagi and sympathetic nerves were uninjured. 



For the protection of the master organ, the brain, 

 there has been evolved the most adequate of structural 

 protections namely, the skull. In accordance with 

 the premise that contact ceptors have been developed 

 only in those parts of the body that have been exposed 

 to the environment, one would expect to find none 

 within the brain, which has always been shielded by its 

 bony covering. To test this point, the brains of 

 anesthetized dogs were exposed and one hemisphere 

 of each was subjected to trauma. No change in 

 blood-pressure or respiration was noted in any instance ; 

 and microscopic examination of the uninjured hemi- 

 spheres showed no histologic changes. So complete 

 throughout phylogeny has been this protection of 



