170 Mr Nasmyth on the Human Mouth. 



both jaws may be considered as concentric arches. The arch 

 formed by the edges of the teeth in the upper, jaw being pro- 

 duced from a little longer radius than that formed by the 

 edges of those in the under, it is evident that if these two 

 arches are forcibly brought into approximation, the external 

 arch of the superior jaw, with its contents, must yield out- 

 wardly ; because, by forcibly applying the crown of an arch 

 to the internal portion of another arch, you obviously afford 

 to the internal arch an incontrollable mechanical advantage. 

 It is also evident, that the forcible retention of any substance 

 between these two arches must increase the intensity of the 

 mechanical advantage, and the tendency of the lower to 

 evert the upper. If we reflect on the peculiar anatomy of 

 the parts, it will be seen, too, that the superior jaw yields 

 to a much greater extent than the inferior. The median 

 suture of the arch of the under jaw is soon consolidated ; 

 whereas there remains a permanently ununited suture in the 

 upper. But its plasticity is still further provided for, and in 

 a more efficient way, by the presence of the intermaxillary 

 bones, which, as their name implies, are situated in the centre 

 between the bones of the true maxilla. These bones have not 

 been generally recognized as separate existences in the adult 

 human subject, though it is universally admitted that they are 

 present in infants, and that they are occasionally to be found 

 distinct throughout life. Some authors have even asserted that 

 the absence of these bones forms a characteristic. But they 

 are of practical importance; and although, if carefully searched 

 for, may be recognized throughout life, yet it is quite sufficient 

 for my present purpose to know that they are recognized in 

 early life, as that is the period at which the characteristic fea- 

 tures are given to the osseous framework, and which conti- 

 nues to the end of our earthly existence. Separate centres of 

 ossification are to be met with here ; and the radiations of 

 these ossific growths are directed to the maxillary bones on 

 each side, the median suture dividing them on the median 

 line. The transverse suture runs almost directly across the 

 palate from the centre of the one alveolar process of the 

 canine tooth to the other, comprehending, in that manner, the 

 whole of the anterior region of the dental range, and impli- 



