270 THE MECHANICS OF THE DIGESTIVE APPARATUS. 



and the half-shadow apparatus of Laurent, Lippich, and others the text-books 

 on physics and chemistry should be consulted. 



THE MECHANICS OF THE DIGESTIVE APPARATUS. 



The mechanism of the digestive apparatus comprises : 



1. The prehension of the food, the movements of mastication and 

 of the tongue, insalivation, and the formation of the bolus. 



2. The movements of deglutition. 



3. The movements of the stomach and the small and large intestines. 



4. The expulsion of fecal matter. 



THE PREHENSION OF FOOD. 



Liquid food is taken into the mouth (i) by suction. While the lips 

 are applied hermetically about the utensil containing the fluid, the 

 tongue, moving downward and at the same time flattened, often in 

 conjunction with depression of the lower jaw, causes the fluid to enter 

 the buccal cavity. Herz found that the negative pressure produced 

 by the suction of infants equals from 3 to 10 mm. of mercury. 

 (2) The liquid is sipped when it is brought directly in contact with 

 the lips, and then is drawn by aspiration into the buccal cavity, 

 together with air, with a characteristic sound. (3) Liquid can also gain 

 entrance into the buccal cavity by being poured, the lower lip, as a rule, 

 being applied to the containing vessel. 



Among the solid articles of food, the smaller particles, supported 

 by the lips, are picked up by the tongue; of the larger particles a piece 

 is bitten off by the chisel-shaped incisor and sharp canine teeth, and 

 then, for further comminution, it is brought between the rough surfaces 

 of the bicuspid and molar teeth. 



THE MOVEMENTS OF MASTICATION, 



The articulation of the lower jaw is divided into two cavities, one above the 

 other, by an interarticular cartilage, which also fulfils the duty of preventing 

 mutual direct pressure of the articular surfaces during the energetic action of the 

 muscles of mastication, in the act of chewing. The articular capsule, considerably 

 strengthened by the external ligament particularly, is so capacious as to permit, 

 in addition to elevation and depression of the lower jaw, also of displacement of 

 the head of the inferior maxilla forward upon the articular tubercle, although the 

 meniscus does not leave the head of the bone, which it covers like a cap. 



The movements of mastication include: (a) Elevation of the jaw, 

 which is effected by the united action of the temporal, masseter and 

 internal pterygoid muscles. If the inferior maxillary bone had pre- 

 viously been greatly depressed, so that the condyles of the bone were 

 moved forward upon the articular eminences, they now drop back 

 into the articular cavity. 



If, in raising the lower jaw, the bone is maintained in a particular position, 

 the action of the muscle that would move the maxilla from this position is lost, 

 as is shown by the following: (i) In elevating the lower jaw when it is pushed as 

 far forward as possible, the action of the temporal muscles is lost, because these, 

 in raising the jaw, draw it backward at the same time. (2) When the lower jaw 

 is pushed as far backward as possible, the temporal muscles alone exert an ele- 



