304 



MOVEMENTS OF THE TONGUE. 



The permanent teeth succeed the milk-teeth, the process beginning about the 

 seventh year. Ten teeth in each jaw take the place of the milk-teeth, while six 

 teeth appear further back in each jaw. Thus the total number of permanent 

 teeth is thirty-two. As the sacs, from which the permanent teeth are developed, 

 are formed before birth, they merely undergo the same process of development as 

 the temporary teeth, only at a much later period. The last of the permanent 

 molars the wisdom-tooth may not cut the jaw until the seventeenth to the twenty- 

 fifth year. At the sixth year the jaw contains the largest number of teeth, as 

 all the temporary teeth are present, and, in addition, the crowns of all the per- 

 manent teeth, except the wisdom-tooth, making forty-eight in all. 



[Eruption of Permanent Teeth. The age at which each tooth cuts the gum ia 

 given in years in the following table : 



(Kirkes.)] 



155. Movements of the Tongue. 



The tongue being a muscular organ (Aretaeus, A.D. 81), and 

 extremely mobile, plays an important part in the process of mastica- 

 tion : (1) It keeps the food from passing from between the molar 

 teeth. (2) It collects into a bolus the finely-divided food after it is 

 mixed with saliva. (3) When the tongue is raised, the bolus lying on 

 its dorsum is pushed backwards into the pharynx, whence it passes 

 into the oesophagus. 



The muscular fibres of the tongue run in three directions longi- 

 tudinally, from base to tip ; transversely, the fibres for the most part 

 proceeding outwards from the vertically placed septum linguae ; vertically, 

 from below upwards. Some of the muscles are confined to the tongue 

 (intrinsic), while others (extrinsic), are attached beyond it to the hyoid 

 bone, lower jaw, to the styloid process, and the palate. 



Microscopically, the fibres are transversely striated, with a delicate sarcolemma, 

 and very often they are divided where they are inserted Into the mucous membrane 

 (Leeuwenhoek). The muscular bundles cross each other in various directions, 

 and in the interspaces fat cells and glands occur. 



On analysing the lingual movements, we may distinguish changes 

 in form and changes in position : 



