THE ORGANS OF THE INNER GERM-LAYER. 313 



The shedding of the teeth ordinarily begins in the seventh year. It 

 is initiated by the disorganisation and absorption of the roots of the 

 milk-teeth, under the pressure of the growing new generation. One 

 finds here exactly the same appearances as in the atrophy of osseous 

 tissue, concerning which we have the thorough investigations of 

 KoLLiKBR. There arise on the roots of the teeth the well-known 

 pits of HowsHiP, in which large, n^ultinuclear cells, the osteoclasts or 

 bone-destroyers, are imbedded. The crowns are loosened by surren- 

 dering their union with the deeper connective-tissue layers. "Finally, 

 when the permanent teeth, owing to the growth of their roots, push 

 forth out of the alveoli, the crowns of the milk-teeth are thereby 

 raised up and fall off. 



The permanent teeth genei-aUy appear in the followiny ordeir : at 

 first, in the seventh year, the first [front] molars ; a year later the 

 middle incisors of the lower jaw, which are followed a little later by 

 those of the upper jaw ; in the ninth year the lateral incisors are 

 ■cut, in the tenth year the first premolars, in the eleventh year the 

 second premolars. Then in the twelfth and thirteenth years the 

 canines and the second molars come through. The eruption of the 

 third molars, or wisdom teeth, is subject to great variation : it may- 

 take place in the seventeenth year, but it may be delayed till the 

 thirtieth. Occasionally the wisdom teeth never attain a complete 

 ■development, so that they are never cut. 



B. The Or gams arising from the Pharynx : Thymus, Thyroid GUmd, 

 LaryrvK, and Lung. 



Whereas in the water-breathing Vertebrates the visceral clefts 

 remain throughout life and subserve respiration, they are completely 

 closed in all Amniota as well as in a part of the Amphib'a. The 

 only exception is in the case of the first cleft, lying between the man- 

 <iibular and the hyoid arches, which is converted into the drum of the 

 ear (tympanum) and the Eustachian tube, and thus enters into the 

 service of the organ of hearing, in connection with which it will 

 subsequently engage our attention. 



However, the remaining visceral clefts do not disappear without 

 leaving any trace. From certain epithelial tracts of these there 

 arises an organ of the neck-region which functionally is still proble- 

 matic, the thymus, the morphology of which has been very essentially 

 a,dvanced during the last few years. 



