174 Mr Nasrayth on the Human Mouth. 



come necessary. There is an accumulation of effect in par- 

 ticular directions, occasionally discoverable, which produces 

 aberrations so extensive that they cannot be explained but 

 upon the admission of the principle of hereditary transmission. 

 Thus, in what I have stated, and in what may follow, it is not 

 to be understood that the effects described as occurring are to 

 be attributed entirely to the exercise of the functions of the 

 parts during one generation, but as being the result of a suc- 

 cession. What the appreciable effect in one generation may 

 be it is impossible to determine upon the data which we at 

 present possess. 



If it be fully confirmed that the mouth of the infant Negro 

 is not prominent, it will be interesting to study the extent of 

 the hereditary influence, and the period of development of 

 that influence. I have hitherto alluded principally to the cir- 

 cumstances attending the development of the anterior portion 

 of the mouth, including the incisors and canines; but charac- 

 teristic habits of different races produce also corresponding 

 deviations in its posterior region. A crowded state of the 

 teeth, from want of due expansion and development of the 

 bcHies in which they are implanted, producing an irregular 

 pressure of one against another in the progress of growth ; 

 and a faulty organization of the dental tissues, increased by 

 that irregularity, are amongst the effects of constitutional in- 

 activity, depending on the habits of social life. But there is 

 one serious evil which is only shewn in social life ; and that is, 

 the derangement interfering with the functions of the mouth, 

 which is occasioned by the arrested development of the jaw, 

 causing a deficiency of room for the development of the wis- 

 dom tooth. This, at times, causes great distress ; and even 

 death, by a slow process of torture. If that tooth at last 

 struggles into external existence under such difficulties, it is, 

 in a great majority of instances, found to be worthless, and 

 only a source of torment to its possessor. On the other hand, 

 however, we find that the rude uncivilized tribes of mankind 

 possess a bold, well-developed, and healthy organization of 

 structure in all the parts, and free from irregular pressure. 

 The wisdom tooth in them is so well developed, free in its 

 position, and healthy in its structure, as to have induced some 



