ORAL DIGESTION. 563 



great disadvantage; the lower jaw representing a lever of the third 

 kind; the fulcrum being in the joint; the power at the insertion of the 

 levator muscles ; and the resistance in the substance between the teeth. 

 The arm of the resistance is, consequently, the whole length of the 

 lever; and we can understand why we are capable of developing so 

 much more force, when the resistance is placed between the molares; 

 and why old people, who have become toothless, and are, consequently, 

 constrained to bite with the anterior part of the jaws, the only portion 

 that admits of contact, cannot bite with any degree of strength. 



The size of the body, put between the incisor teeth, influences the 

 degree of force that can be brought to bear upon it. When small the 

 force can be much greater, as the levator muscles are inserted perpen- 

 dicularly to the lever to be moved, and the whole of their power is 

 advantageously exerted; but if the body be so large, that it can scarcely 

 be received into the mouth, and be resisting withal, the incisors can 

 scarcely penetrate it ; the insertion of the levator muscles into the 

 jaw being rendered very oblique ; and the greater part of the force they 

 develope consequently lost. This will be readily seen by Figure 247. 

 When the mouth is closed, or nearly so, the masseter, and temporal 

 muscles represented respectively by the lines B E and J /, are inserted 

 nearer the perpendicular ; but when the lower jaw is depressed, so that 

 the situation of these muscles is represented by the dotted lines B e and J 

 &, the direction in which the muscles act will be more oblique, and, there- 

 fore, more disadvantageous. When the muscles of the jaws are incapa- 

 ble, of themselves, of separating the substance, as in the case of the 

 apple, the assistance of the muscles of the hand is invoked ; whilst the 

 muscles on the posterior part of the neck, which are inserted into the 

 head, draw it backwards; and, by these combined efforts, the substance 

 is forcibly divided. 



c. Oral or Buccal Digestion. 



The changes, effected upon the food in the mouth, are important 

 preliminaries to the function that has to be executed in the stomach 

 and duodenum. As soon as it enters the cavity, it is subjected to the 

 action of the organ of taste; and its sapid qualities are appreciated. 

 By its stay there, it also acquires nearly the temperature of the cavity. 

 This is, however, a change of little moment, unless the food is so hot, 

 that it would injure the stomach, if passed rapidly into it. Under such 

 circumstances, it is tossed about in the mouth, until it has parted with- 

 its caloric to various portions of the parietes of the cavity; and then, 

 if in a fit state for the action of deglutition, it is transmitted along the 

 oesophagus; but the most important parts of oral digestion are the 

 movements of mastication and insalivation by which solid food is com- 

 minuted, and imbued with the secretions poured into the interior of the 

 mouth, and which we have shown to be of a very compound character. 



Under the sense of taste, the influence of the agreeable or disagreea- 

 ble character of the food upon the digestive function was expatiated 

 upon. It is unnecessary, therefore, to do more than allude to the sub- 

 ject here. We find that whilst a luscious aliment excites to prolonged 

 mastication, and the salivary glands to augmented secretion, the mas- 



