ORGANS OF Till I 313 



The sfidt'/iiii/ a/ (/! f"-f/i onlinariU t the sevei li 



is initiated by tin- (li->oi- u r :ini-:it ion ami absorption of the roots of the 

 inilk-terth, under tl ire of tli.- Lrrowini: new genor.it i-.n. One 



limls lit -re rxactly tin- same appearances as in the atrophy of osseous 

 ti->ue, concernim: which we have the thorough investigation 

 KOLLIKER. There arise on the roots of the teeth the well kno\\n 

 pits of Ilowsim-, in which hr<:e. mult inucloar cells, the oateoclasts or 

 bone-destroy era, are imbedded. The crowns are loo>ened by surren- 

 derini: their union with the deeper connective-tissue layer-. l-'inally, 

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

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

 rai>ed up and fall off. 



The permanent teeth generally <t/>/'>ir in the /vlltnriny order : 

 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 Organs arising from the Pharynx : Thymus, Thyroid Gland, 

 Larynx, and Lv 



Whereas in the water-breathing Vertebrates the visceral c! 

 remain throughout life and subserve respiration, they are completely 

 1 in all Amniota as well as in a part of the Amphibia. The 

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

 dibular and the hyoid arches, which is con verted into the drum of the 

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

 >ervice of the organ of hearini:, in connection with which it will 

 Mil>M>4iie:itly engage our attention. 



However, the remaining vixvi.il defts do not disappear without 

 having any trace. From certain epithelial tracts of these there 

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

 matic, the thymus. the morphology of which has been very essentiallv 

 advanced during the last few yen 



