THE MECHANICS OF DIGESTION 1001 



swallowed in rather large masses, whereas in the herbivora, and especially 

 in the ruminating mammals, it is slowly reduced into the smallest 

 possible fragments. The omnivora, such as man, occupy an interme- 

 diate position in this regard. These differences are associated with 

 definite peculiarities in the shape and structure of the parts concerned 

 in mastication. Thus, we find that the teeth of the carnivora are 

 well adapted to catch the food, while those of the ruminants present all 

 the characteristics of grinders. In man, the incisors are to hold and 

 to divide the food, whereas the canines divide it, and the bicuspids 

 and molars macerate it. The development of these parts proceeds 

 in the same manner as that of the hairs. A continuous thickening of 

 the epithelium takes place along the gums which grows into the corium 

 of the mucosa and forms the dental germ or dental lamina. Further 

 thickenings and growths give rise to the special dental germ from which 

 the milk teeth are developed. Each germ contains a vascular papilla 

 and is eventually separated from the general mucous membrane by a 

 vascular septum, which is known as the dental sac. The papilla is 

 finally transformed into the dentine and pulp of the growing tooth, 

 while its enamel is deposited upon this core by the epithelial cells of the 

 dental germ. Later on, as the tooth grows outward, its root is formed 

 which is then covered with cement. 



In man, the teeth appear in two sets, a temporary one and a permanent one. 

 The first consists of the so-called milk teeth. They are twenty in number and appear 

 between the 5th and 30th month. Their time of appearance, however, varies 

 considerably, being subject to family characteristics, and the condition of the child. 

 The first to appear are the two central incisors below (5th to 9th month), next the 

 four upper central teeth (8th to 12th month) ; then the other two lower central 

 teeth and the four front double teeth (12th to 18th month). The four incisors follow 

 next (18th to 24th month), the upper being known as the "eye teeth" and the 

 lower as the "stomach teeth." The four back double teeth which complete the 

 first set, break through between the 24th and 30th month. Every one of the 

 milk teeth is replaced in the course of time by a permanent tooth. This change 

 begins at about the 7th year and proceeds in about the same sequence as the 

 formation of the temporary set. In addition, each maxilla acquires six new teeth, 

 three on each side. These are the permanent molars. The last of these, or wis- 

 dom teeth, appear about the 20th year, but have been known to be delayed until 

 the 30th year and later. The permanent set, therefore, consists of thirty-two 

 teeth. 



Deglutition. — In brief, the process of mastication consists in a 

 mechanical reduction and anointment of the food which eventually 

 leads to the formation of the holus. This rounded pulpy mass of food 

 is then projected into the stomach by the process of deglutition or 

 swallowing. In general, it may be said that the onward movement of 

 the food through the alimentary canal is effected by peristaltic motion, 

 but the gross character of this muscular activity differs somewhat in 

 the different segments of this channel. The act of deglutition is 

 divided into three stages. The first is oral in its character and termi- 

 nates with the passage of the bolus through the pillars of the fauces. 

 The second concerns the constituents of the pharynx and ends with 



