166 Mr Kasmyth on the Human Mouth. 



plastic nature as to admit of this factitious development. 

 Their habits and exactions will also require to be considered 

 for the purpose of ascertaining how they become plastic, and 

 are factitiously modelled out of their congenital arrangement ; 

 and, with a view to understand the nature and extent of the 

 plasticity of the osseous portion of the organ, I shall now 

 describe the anatomy of the mouth, and shew how far these 

 parts are under the influence of the moulding and controlling 

 powers of the muscles, in the performance of the functions 

 required of them. As I do not, however, intend to give here 

 a strictly anatomical demonstration, nor yet a physiological 

 disquisition, what I shall say will consist more of a general 

 explanation of that which is necessary to be attended to, 

 with a view to understand my theory, than anything else. 

 I have already alluded to .the complicated nature of the oper- 

 ations exacted of the mouth in articulation and mastication. 

 The degree of perfection in the development of all the differ- 

 ent portions of the mouth, must regulate the degree of per- 

 fection with which the work to be performed by it is accom- 

 plished. Perfectly distinct articulation is not compatible 

 with the prominent jaw of the uncivilised, neither is it com- 

 patible with the irregularly-developed mouth of the civilized ; 

 nor is it possible for the diversified exercise of the organ in 

 the different actions exacted for the division, comminution, 

 and grinding of food to be well performed by such mouths, 

 as social life is every day furnishing in endless variety. The 

 irregularity of the teeth in such mouths, causes the one jaw 

 to become locked within the other, and thereby prevents 

 such latitude of action as is adequate to the due performance 

 of these varied duties. MasticMion is performed by means 

 of one portion of the mouth being passive, and the other ac- 

 tive; the under jaw, consisting of bone and muscle, is the 

 active, the upper jaw the passive portion ; but although, 

 collectively, the under jaw is active, yet 'this is again re- 

 solvable into a single portion acted upon, namely, the solid 

 bone and a number of parts producing the action, or in 

 which the power resides, namely the muscles. The force 

 exerted is that of a lever of the third order, the principal 

 force being exerted by the powerful temporal muscles in- 



