PULPS AND SACS OF THE HUMAN TEETH. 51 



5. Dentition commences in the upper jaw, and continues 

 in advance during the most important period of its progress. 

 The first tooth-germ which appears is that of the superior 

 anterior molar, which precedes that of the inferior anterior 

 molar. 



The apparent exception to this law in the case of the in- 

 ferior incisive has already been explained. 



Permanent Teeth. 6. The germs of the permanent teeth, 

 with the exception of that of the anterior molar, appear in a 

 direction from the median line backwards. 



7. The milk-teeth originate, or are developed, from the 

 mucous membrane. 



8. The permanent teeth, also originating from mucous 

 membrane, are of independent origin, and have no connection 

 with the milk-teeth. 



9. A tooth-pulp and its sac must be referred to the same 

 class of organs as the combined papilla and follicle from which 

 a hair or feather is developed viz. bulbs.* 



* An abstract of this paper was read at the last meeting of the British 

 Association for the Advancement of Science. 



Dr. Allen Thomson stated to me at that time, that he had no doubt that 

 the fact of the milk-tooth sacs being at one period open follicles had been ob- 

 served, but that, then, he could not inform me where I could find it mentioned. 

 I saw Dr. Thomson in Edinburgh a few weeks afterwards, when, on looking 

 into Valentin's work on Development (ffandbuch der EntwicJcelungs-ge- 

 schichte des Menschen), he pointed out to me the fact that Arnold had observed 

 that the milk-tooth sacs were formed by a duplicature of the mucous mem- 

 brane of the mouth, and had inserted a notice of the discovery in the Salzburg 

 Med. Chir. Zeitung, 1831, p. 236. In order that Professor Arnold's discovery 

 (which appears to have been altogether overlooked both in this country and in 

 France) may be more generally known, I will give all his facts as he has re- 

 corded them. His notice occupies less than a page, and I am not aware that 

 he has extended it elsewhere. At p. 236, loc. cit. he has observed, "In an 

 embryo at the ninth week, we may perceive in both jaws, on the projecting 

 edges of the gums, a proportionally pretty deep furrow, with ten depressions 

 in it ; a little later we may see a flat surface, on which there are many open- 



