DIGESTION. 51 



forward, and laterally by acting on the bone in which they are im- 

 planted. By the motion of the lower teeth upon the upper the food 

 is comminuted. During mastication the food is mixed with the 

 saliva and fluids of the mouth, which latter cavity is closed ante- 

 riorly and posteriorly during the process. 



The disintegration of the food by mechanical reduction, is mani- 

 festly aided by insaUvation ; and the admixture of saliva appears 

 further, to have the effect of commencing the transformation of the 

 starchy particles of the food into sugar. From experiments of Ber- 

 nard and Barreswil, it appears that saliva, when acidulated, possesses 

 the same power of acting on azotised compounds as that which cha- 

 racterizes gastric juice ; consequently when introduced into the sto- 

 mach, the saliva may afford important aid in the digestive process. 



Deglutition. — The food, comminuted and moistened in the mouth 

 by the means above mentioned, is prepared for the action of deglu- 

 tion. In this there are three stages. In the^rs^, the particles, of the 

 food, collected to a bolus, glide between the surface of the tongue 

 and the palatine arch, till they have passed the anterior arch of the 

 fauces. This is a purely voluntary movement. In the second^ the 

 bolus is carried past the constrictors of the pharynx. In the thirds 

 it reaches the stomach through the oesophagus. These three acts 

 follow each other with extreme rapidity. 



Durino; the second stag-e of deg-lutition, the tongue, the muscles of 

 the anterior and posterior half arches, the superior muscles of the 

 soft palate, and the constrictors of the pharynx, are all in action. In 

 this stage, by the retraction of the tongue, and the elevation of the 

 larynx, the epiglottis is pressed over the rima glottidis, which is also 

 closed during this process. 



The communication between the fauces and posterior nares is cut 

 off by the muscles of the posterior palatine arches, which contract 

 in such a manner, as to cause the sides of the arch to approach each 

 other like a pair of curtains, and to the cleft between the two sides, 

 the uvula is applied like a valve. Some of these acts may be per- 

 form^ voluntarily, but the combination of the whole is automatic, 

 and under the presidency of the reflex system of nerves. 



In the third act, in which the food passes through the oesophagus, 

 every part of that tube as it receives the bolus, and is dilated by it, 

 is stimulated to contract. 



The movements of the oesophagus are entirely involuntary and 

 rhythmical in their character ; in the act of vomiting they are in- 

 verted. At the point where the cesophagus enters the stomach, — the 

 cardiac orifice of the latter, — there is' a sort of sphincter. This 

 opens when there is sufficient pressure made upon it by the accumu- 

 lated food, and afterwards closes so as to retain the food in the 

 stomach. The opening of the cardiac orifice is one of the first acts 



