Mr Nasniyth on the Human Mouth. 171 



<5ating, in their development, the centre of activity of the 

 central region of the arch. They thus affect, by their pres- 

 sure, such a form of arrangement as to admit of a great plas- 

 ticity in the anterior arch of the mouth. 



I have already briefly adverted to the ordinary duties re- 

 quired of the teeth situated in the anterior portion of the 

 mouth ; and a moderate exercise of these may be considered 

 their particular duties in a somewhat advanced stage of culti- 

 vated human existence. If these teeth are duly exercised, and 

 proportionately with the others, we have then a develope- 

 ment properly fitted for all social requisitions, at once afford- 

 ing the power of perfect articulation and perfect mastication. 

 Articulation is entirely performed in this region of the mouth ; 

 and although mastication, properly so called, is not performed 

 here, yet it is materially interfered with by any deviation from 

 a regular arrangement even in this quarter. Thus, where 

 there is an excess of luxury and indolence in social life, we 

 find, from the want of functional exercise, that the jaws are 

 not duly developed, and that early anchylosis of the different 

 sutures is the result. From the osseous portion not being pro- 

 perly developed, space is not afforded for the accommodation 

 of the second set of teeth. The second or permanent teeth, 

 in the early stage of infantile existence, are arranged in the 

 jaw behind the temporary teeth, and, consequently, in the arch 

 of a circle of a shorter radius than that in which their prede- 

 cessors are placed ; and, being of larger dimensions, and con- 

 fined within a smaller compass, they are forced to overlap each 

 other in the very early and unextruded state. The indolence 

 of the system will thus permit these teeth to creep into exter- 

 nal existence in foetal arrangement, and they really do appear 

 in that condition in the heads of a great proportion of the 

 adults of civilized life. In such cases, unless corrected by art, 

 the mouth approximates a carnivorous type, and inertness of 

 comminution or grinding in the posterior or true masticating 

 region is the consequence, from the impossibility of using the 

 jaws in such an operation. Articulation, also, is sometimes 

 materially interfered with from the unequal surface produced 

 by the irregular arrangement of the anterior teeth upon 

 which the tongue has to act. These irregularities of arrange- 



