174 DIGESTION. [CHAP. xxm. 



the teeth, and which there form a plexus in the substance of a firm 

 elastic tissue, which is the connecting medium between the socket 

 and the root, and is usually regarded as a periosteum. Thus the 

 surface of the root is supplied with blood, and the tooth is united 

 to the jaw in a way which allows it to yield very slightly under 

 pressure. 



The alveolar arch is covered on the outside by the gums, a dense, 

 elastic, peculiar tissue, adapted to sustain without injury the forcible 

 contact of the hard portions of food, to which its vicinity to the 

 grinding organs must expose it. 



The development of the teeth may be described under two heads : 

 first, that of the elementary tissues of the tooth, the dentine, the 

 enamel, and the true bone; secondly, that of the dental series, 

 which will include the order of appearance of the teeth of the tem- 

 porary and permanent sets. 



According to the most recent investigations of Arnold and Good- 

 sir, the teeth are developments from the mucous membrane cover- 

 ing the dental arches, and not from the maxillary bones. They, there- 

 fore, would seem to correspond with the tegumentary appendages 

 of animals, such as horns, nails, feathers, and not to be a portion 

 of the true osseous system, or endo-skeleton of the vertebrata. 

 The bills of birds are an obvious intermediate condition of the in- 

 tegument of the jaws. 



The teeth may be regarded as formed in the following manner. 

 The first indication observed (according to the excellent observa- 

 tions of Mr. Goodsir, which we have in most particulars had an 

 opportunity of verifying) is a groove at the border of the palate 

 in the situation of the future teeth, which he terms the primitive 

 dental groove, and which is apparent in the foetus of six weeks old. 

 At the bottom of this groove there appear certain fine papillae, 

 which increase in size, and gradually assume the shape of the 

 crowns of the future teeth, having the edge and cusps which are 

 eventually to distinguish them. As the papillae grow, the groove 

 is converted into follicles for their reception, by the growth of septa 

 between its borders, that is, between the outer and inner alveolar 

 processes, which soon begin to be ossified within the walls of the 

 groove. Thus the follicles become alveoli lined by the periosteum 

 of the jaw, such as it is at this early period, and lodging a process 

 of the mucous membrane of the gum, from the bottom of which 

 springs up the papilla or germ of the future tooth. The summit of 

 the papilla is at first visible in the mouth of the follicle ; but ere it 

 has assumed the figure of the tooth, the margins of the orifice enlarge 



