consider the saliva as the _principal_ agent in digestion, have not a shadow of foundation for their opinion. When unmasticated food is introduced into the stomach, the gastric juice acts only upon its surface, and changes of a purely chemical nature sometimes commence in its substance before its digestion can be effected. Hence often arise, especially in children, those pains and troubles, that nausea and acidity, consequent on the continued presence of undigested aliment in the stomach. By a peculiarity of constitution, however, the stomach will not long retain food which it cannot dissolve. After a number of hours,--varying, according to the state of health, from one or two to ten, or even twenty,--it is either rejected by vomiting, or transmitted unchanged to the intestine, where its presence irritates and gives rise to colic, flatulence, bowel-complaints, and, in delicate children, not unfrequently to convulsions. Hence another proof of the importance of slow and deliberate mastication. * * * * * As soon as the morsel has been thoroughly masticated and impregnated with saliva, it is ready for transmission to the stomach. To this part of the process the term _deglutition_ or _swallowing_ is applied. Immediately at the back part of the mouth several passages present themselves, leading in different directions--one upwards and forwards into the nose, another downwards and in front into the windpipe, and a third downwards and behind into the _œsophagus_, _gullet_ or _meat-pipe_ and stomach. The last is the passage taken by the food, and the violent coughing and occasional suffocation induced when it accidentally passes into the windpipe, are but a specimen of the serious evils which would be continually occurring if some provision were not made to obviate the danger, while the rarity with which such accidents actually happen, proves the almost unfailing efficacy of that which has been devised.