MASTICATION. 189 



so that the lips are kept covered with the liquid and are closed around the 

 edge. By a gentle, sucking action the liquid is then introduced. This is 

 the ordinary mechanism of drinking ; but sometimes the head is thrown back 

 and the liquid is poured into the mouth, as in " tossing off " the contents of 

 a small vessel. 



MASTICATION. 



In order that digestion may take place in a perfectly natural manner, it 

 is necessary that the food, as it is received into the stomach, should be so far 

 comminuted and incorporated with the fluids of the mouth as to be readily 

 acted upon by the gastric juice ; otherwise, gastric digestion is prolonged and 

 difficult. Non-observance of this physiological law is a frequent cause of 

 what is generally called dyspepsia. 



Physiological Anatomy of the Organs of Mastication. In the adult, each 

 jaw is provided with sixteen teeth, all of which are about equally developed. 

 The canines, so largely developed in the carnivora but which are rudimentary 

 in the herbivora, and the incisors and molars, so fully developed in the her- 

 bivora, are, in man, of nearly the same length. Each tooth presents for ana- 

 tomical description a crown, a neck and a root, or fang. The crown is that 

 portion which is entirely uncovered by the gums ; the root is that portion 

 embedded in the alveolar cavities of the maxillary bones ; and the neck is 

 the portion, sometimes slightly constricted, situated between the crown and 

 the root and covered by the edge of the gum. Each tooth presents, on sec- 

 tion, several distinct structures. 



Enamel of the Teeth. The crown is covered by the enamel, which is by 

 far the hardest structure in the economy. This is white and glistening and 

 is thickest on the lower portion of the tooth, especially over the surfaces 

 which, from being opposed to each other on either jaw, are most exposed to 

 wear. It here exists in several concentric layers. The incrustation of enamel 

 becomes gradually thinner toward the neck, where it ceases. The enamel is 

 made up of pentagonal or hexagonal rods, one end resting upon the subjacent 

 structure, and the other, when there exists but a single layer of enamel, ter- 

 minating just beneath the cuticle of the teeth. 



The exposed surfaces of the teeth are still farther protected by a 

 membrane, -jyj-^ to 15 i o of an inch (O8 to 1-7 ft) in thickness, closely 

 adherent to the enamel, called the cuticle of the enamel (Nasmyth's mem- 

 brane). The cuticle presents a strong resistance to reagents and is useful in 

 protecting the teeth from the action of acids which may find their way into 

 the mouth. 



Dentine. The largest portion of the teeth is composed of dentine, or 

 ivory. In many respects, particularly in its composition, this resembles bone ; 

 but it is much harder and does not possess the lacunae and canaliculi which 

 are characteristic of the true osseous structure. The dentine bounds and 

 encloses the central cavity of the tooth, extending in the crown to the enamel, 

 and in the root, to the cement. It is formed of a homogeneous, fundamental 

 substance, which is penetrated by a large number of canals radiating from 

 14 



