FORMATION OF THE TISSUES TEETH. 491 



branches, which again send off smaller ones. In some animals, these tubuli 

 may be traced at their extremities into cells exactly resembling the bone-cavi- 

 ties; and here the Ivory must be considered as having undergone a transition 

 into the substance next to be described. The diameter of the tubuli in their 

 largest part averages about 1 -10,000th of an inch ; their smallest branches are 

 immeasurably fine. It is impossible that they can receive blood ; but it may 

 be surmised that, like the tubuli of bone, they absorb matter from the vascular 

 lining of the pulp-cavity, which aids in the nutrition of the tooth. Although, 

 when once fully formed, the Tooth undergoes little or no change, there is evi- 

 dence that it possesses a certain power of repairing the effects of disease, a 

 new layer of hard matter being sometimes thrown out on a surface which has 

 been laid bare by Caries. It has been found, too, that the Dentine is sometimes 

 tinged by colouring matters contained in the blood. This is most evident, 

 when a young animal is fed upon madder, during the period of the formation 

 of the tooth ; but even in an adult, some tinge will result from a prolonged use 

 of this substance ; and it has been noticed that the teeth of persons who have 

 long suffered from Jaundice sometimes acquire a tinge of bile. Some uncer- 

 tainty exists in regard to the organization of the animal matter of Dentine. 

 Appearances have been seen by Mr. Owen, especially in the tooth of the Du- 

 gong, which appear distinctly to indicate, that the cellular structure of the pulp 

 may be to a certain degree persistent in the Dentine ; and he regards the 

 tubuli as formed from the nuclei of these cells, as are the stellate cavities of 

 bone. On the other hand, it is stated by Henle that, in the cartilaginous basis 

 of Dentine, a distinctly fibrous structure prevails ; each of the tubuli being 

 surrounded with a fasciculus of flat pale granular fibres. The Cartilaginous 

 basis obtained by the action of acid resembles that of Bone in its composition, 

 but only forms about 28 parts in 100 ; the phosphate of lime constitutes about 

 64 5 parts, carbonate of lime 5 parts ; phosphate of magnesia and soda, and 

 chloride of sodium, 2| parts. The Crusta Petrosa or Cementum has the 

 characteristic structure of true Bone ; and does not seem to differ from it in 

 any essential particular. Besides these three substances, a fourth has been 

 described by Mr. Nasmyth, as occurring normally in many of the lower ani- 

 mals, and as occasionally presenting itself in Man. This is formed by the 

 consolidation of the central part of the pulp, which usually remains uncal- 

 cified, occupying the centre of the tooth : it " partakes much of the fibrous 

 character of the ivory, being composed of irregularly radiating filaments, 

 blended with small calcigerous cells, in which the ossified vessels are seen to 

 ramify ;" and may be considered, therefore, as intermediate between true Ivory 

 and Bone. By Mr. Nasmyth, the occurrence of this substance in the Human 

 Tooth was regarded as the result of diseased action ; but it has been pointed 

 out by Mr. Lintott,* that it constantly occurs after the age of twenty, and in- 

 creases with advancing years. 



635. The Development of the Teeth presents many points of great interest ; 

 and will therefore be described in some detail. The following account of the 

 early formation of the teeth in the Human foetus, is derived from the researches 

 of Mr. Goodsir.t 



a. At the sixth week of Foetal life, a deep narrow groove may be perceived, in the 

 upper jaw of the Human embryo, between the lip and the rudimentary palate; this is 

 speedily divided into two by a ridge, which afterwards becomes the external alveolar 

 process; and it is in the inner groove that the germs of the teeth subsequently appear. 

 Hence this may be termed the primitive dental groove. At about the seventh week, an 

 ovoidal papilla, consisting of a granular substance, makes its appearance on the floor of 

 the groove, near its posterior termination; this papilla is the germ of the Anterior supe- 



* Lancet, June 24, 1843. 



f Edinb. Med. and Surg. Journal, vol. li. 



