in DIGESTION IN THE MOUTH AND STOMACH 155 



Fluids, whether taken into the mouth by sucking or drinking 

 (i.e. imbibed or gulped down), are immediately swallowed ; solids, 

 on the contrary, are masticated before swallowing. 



If a fluid is taken up by imbibing, this is effected by the 

 negative pressure which is produced in the buccal cavity during 

 an inspiration, provided the communication between the pharynx 

 and the nasal fossae is closed by elevation of the palate. In 

 order that the liquid shall be imbibed, the edges of the lips 

 must be applied to the edge of the glass and to the surface of 

 the fluid. When a fluid is gulped down, the mouth is half-open, 

 and the lower lip makes a funnel which conducts the fluid to 

 the mouth. 



Sucking, by which the infant draws its nourishment from the 

 glands of the breast, is effected by the vacuum produced in the 

 mouth by depression of- the roof, retraction of the tongue towards 

 the throat, and sometimes by dropping of the lower jaw (Auerbach), 

 while the lips completely enclose the nipple. The negative 

 pressure which determines the flow of milk into the buccal cavity 

 oscillates, according to Herz, between 3 and 10 mm. Hg; this last 

 figure, however, seems to us exaggerated. 



Mastication of solid foods is accomplished by the voluntary 

 movements of the lower against the upper jaw, assisted by the 

 movements of the tongue, by which the food is pushed between 

 the two rows of teeth, which are the passive instruments of 

 trituration ; the canines and incisors serve particularly for pulling 

 and tearing, the molars for biting up the food. 



The teeth make their appearance in the first two years of 

 childhood these being the milk teeth destined to be gradually 

 replaced by the permanent teeth from the seventh year onwards. 

 The milk teeth are 20 in number : 8 incisors, 4 canines, 8 molars ; 

 the -permanent teeth are usually 32 : 8 incisors, 4 canines, 8 pre- 

 molars, and 12 molars. Comparison of the form of the teeth in 

 man and in the carnivora and herbivora, gives a plain anatomical 

 proof that a mixed diet is that best adapted to the nature of man. 

 This is confirmed by the length of his intestine, which holds the 

 mean between that of herbivora, which is much longer, and of 

 carnivora, which is much shorter. 



Elevation of the lower jaw is effected by means of the temporal, 

 masseter and internal pterygoid muscles ; its depression, by gravity, 

 and by the action of the anterior surface of the digastric, mylo- 

 and genio-hyoid muscles, and the platysma ; the forward movement 

 by the simultaneous action of the external pterygoids ; the 

 retraction by the simultaneous movement of the internal ptery- 

 goids ; and the sideway movements by the alternate action of the 

 external pterygoids on both sides (Fig. 52, A and B). 



The masticatory movement is regulated by the tactile sensibility 

 of the teeth and the buccal mucous membrane, and by the 



