THE DEVELOPMENT OF THE TEETH. 145 



But though it exists at this early period, it is not till 

 later, when, after the completion of the dentine and enamel 

 immediately beneath it, its own function is about to come 

 into play, that it attains to its characteristic structure. 

 This M. Magitot designates as fibro-cartilaginous, as there 

 appear in it characteristic cartilage cells or chondroplasts, 

 containing one, two, or rarely three cells, which have 

 spherical or ovoid nuclei. 



In those creatures which have cementum upon the roots of 

 the teeth only, no special cement organ exists, but osteoblasts 

 which calcify into cementum are furnished by the tooth sac. 



It is said that the inner layer of the tooth sac is concerned 

 in the formation of the cement ; that the outer layer, con- 

 jointly with the surrounding connective tissue, is converted 

 into the alveolo-dentar periosteum, but I cannot myself 

 recognise 'the justice of this distinction in practice. In 

 human teeth the parts of the follicle wall or sac cease to be 

 distinctly distinguishable at a comparatively early period, 

 and their importance is not such as to call for very detailed 

 description. 



Another structure, once thought important, and now 

 known to be a mere bundle of dense fibrous tissue, is the 

 " gubernaculum." The permanent tooth sacs, during their 

 growth, have become invested by a bony shell, which is 

 complete, save at a point near their apices, where there is a 

 foramen. Through this foramen passes a thin fibrous cord, 

 very conspicuous when the surrounding bone is broken 

 away, which is called the " grubernaculum," from the notions 

 entertained by the older anatomists that it was concerned 

 in directing or effecting the eruption of the tooth. The 

 gubeniacula of the front permanent tooth sacs perforate 

 the alveolus and blend with the gum behind the necks of 

 the corresponding milk teeth; those of the bicuspids unit- 

 ing with the periosteum of the alveoli of their deciduous 

 predecessors. 



