230 DIGESTIVE SYSTEM. 



the purpose of conveying the drops of saliva into the 

 throat, where their presence excites the reflex action. The 

 will is likewise powerless to prevent deglutition, which latter 

 continues, even if a body dangerous to life is brought in con- 

 tact with this region. Finally, the most remarkable fact is 

 that the act of swallowing must begin at the beginning : if 

 the food be stopped in its course, it can continue only by 

 means of a fresh movement of deglutition, commencing at 

 the isthmus of the fauces. 



The spinal cord is the centre of these nervous phenomena 

 whose centripetal organs are the sensory branches of the tri- 

 geminus, the glosso-pharyngeal, and the superior laryngeal 

 nerves; the centrifugal nerves are the motor branches of 

 the glosso-pharyngeal, and the pneumogastric nerves, re-en- 

 forced by anastomoses with the facial and the spinal nerve. 



The will being powerless to produce this phenomenon, we 

 see that the brain has no share in it : thus the act of swal- 

 lowing may be excited in persons under the influence of 

 narcotics, or in animals whose brain has been removed. 



The region of the isthmus of the fauces may also be the 

 centre of anti-peristaltic movements, accompanied by dis- 

 agreeable sensations (disgust) and causing vomiting (nausea) ; 

 for this reason the glosso-pharyngeal nerve, which appears to 

 be the special conductor of the sensations, is sometimes 

 called the nerve of nausea. 



The normal execution of the reflex action of deglutition 

 also appears to require that the epithelium of the isthmus of 

 the fauces shall be in a sound state. This epithelium often 

 suffers from atrophy, and, on account of this, the sensibility 

 of the part is impaired, and the reflex action consequently 

 rendered difficult or impossible, from failure of the impres- 

 sion which gives rise,to it : this is the reason that, in chronic 

 diseases, we see persons die or, rather, let themselves die 

 of hunger, because the act of swallowing has become too 

 painful. 



The epithelium of the supra-diaphragmatic portion of the 

 digestive canal is generally of great importance in pathology: 

 in some cases of disease it thickens and falls off, causing the 

 furred coating of the mouth. This abnormal growth is only 

 an exaggeration of what takes place in a normal condition in 

 certain parts, in the tonsil, for instance. The base of this 

 organ is formed of elements similar to those of the lymphatic 

 follicles ; but numerous prolongations in culs-de-sac proceed 

 to it from the buccal epithelium, making its structure spongy, 



