42 



Basic Structure of Vertebral* 



The crown of the tooth develops first. As the tooth grows, it in- 

 creases in length as well as in thickness, adding first the neck region and 

 later the root. The opening into the pulp-cavity becomes narrower as 

 the root elongates until finally only a minute foramen remains to admit 

 blood-vessels and nerves. The nerves, branches of the fifth cranial 

 (trigeminal) nerves, grow into the pulp and each terminates in deli- 

 cate fibrils which are intimately related to the odontoblast cells. The 

 layer of cement is the last part of the tooth to develop. The cement is 

 produced by bone-forming cells (osteoblasts) derived from connective 

 tissue external to the original tooth-germ. Eventually the bone of which 

 the jaw is constituted fills in closely around the root of the tooth, form- 

 ing a bony pocket or alveolus in which the tooth is firmly held in place. 



The eruption (i.e., the piercing of the gum and emerging of the 

 crown into the oral cavity) of the milk-teeth of man usually begins in 



ENAMEL PULP 



_— AMELOBLAST LAYER 



— ENAMEL 

 .ig.- DENTINE 

 £- -ODONTOBLAST LAYER 



SB— PULP CAVITY 



TEMPORARY TOOTH 



Fig. 38. A section of the jaw of a nine-month human embryo, showing a 

 developing canine tooth. The enamel-organ of the permanent incisor is seen on the 

 lingual side of the milk-tooth. (After Corning. Courtesy, Neal and Rand : " Chordate 

 Anatomy," Philadelphia, The Blakiston Company.) 



