THE DIGESTIVE SYSTEM 247 



Twenty of the permanent teeth correspond in position to the twenty milk 

 teeth, while twelve new molar germs, six in the upper and six in the lower jaw, 

 represent the true molars of the adult. The first permanent tooth germ to appear 

 is that of the first molar, about the beginning of the fifth month (embryos of 

 about i8o mm.). It lies just behind the second molar of the milk dentition. 

 The germs of the incisors and canines appear about the end of the sixth month, 

 those of the premolars, which replace the milk molars, in the beginning of the 

 eighth month. The germs of the second and third (w-isdom teeth) molars do 

 not appear until after birth, those of the former at about six months, of the 

 latter during the fifth year. 



The CEMENTUM is developed by ossification of that part of the dental sac 

 which covers the root, its development being similar to subperiosteal bone 

 formation (p. 207), without the formation of Haversian systems. 



TECHNIC 



(i) Teeth are extremely difficult organs from which to obtain satisfactory 

 material for study. Sections of hard (undecalcified) and of decalcified teeth 

 may be prepared in the same manner as sections of bone — technics i, and 2 p. 

 202. The decalcified tooth should include if possible the alveolar margin of the 

 jaw, so that in longitudinal sections the mode of implantation and the relation 

 of the tooth to the surrounding structures can be seen. 



(2) For the study of developing teeth, embryo pigs, sheep, cats, dogs, etc., are 

 suitable. For the early stages foetal pigs should be five to six inches long; for the 

 intermediate, ten to twelve inches. The later stages are best obtained from a 

 small new-born animal, e.g., kitten or small pup. The jaw — preferably the 

 lower — or pieces of the jaw are fixed in formalin-jNIiiller's fluid (technic 6, p. 7), 

 hardened in alcohol, and decalcified (page 10). Subsequent treatment is the 

 same as for developing bone (technic i, p. 208). 



The Pharynx 



The wall of the pharynx consists of three coats — mucous, muscu- 

 lar, and fibrous. 



I. The mucous membrane has a surface epithelium and an un- 

 derlying stroma. 



The EPITHELIUM is stratified squamous except in the region of the 

 posterior nares, where it is stratified columnar cihated, continuous 

 with the similar epithelium of the nasal mucosa. 



The STROMA, or tunica propria, consists of mixed fibrous and 

 elastic tissue infiltrated with lymphoid cells. In certain regions 

 these cells form distinct lymph nodules (see pharyngeal tonsils, page 

 184). Beneath the stratified squamous epithelium the stroma is 

 thrown up into numerous low papillce. These are absent in regions 



