SECOND DENTITION. 1023 



under the influence of some new impression or of some change in the direc- 

 tion of the ideas. But the same general principle applies to this case, as to 

 the formation of habits of thought ; namely, that although individual im- 

 pressions are more speedily dissipated from the minds of children than from 

 those of adults, yet that when impressions of the same kind are frequently 

 repeated, the brain grows to them in such a manner that they come to take 

 part (as it were) in its ordinary working; and thus, by establishing a par- 

 ticular mode of nutritive assimilation, they tend to perpetuate this acquired 

 habit, of whatever nature it be. The right training of the Emotional tenden- 

 cies, and all the higher uses of the Intellectual Faculties, depend in great 

 degree, as may readily be shown, upon the influence of the Will in directing 

 the current of thought and feeling ; and this becomes greater and greater, if 

 rightly cultivated, with the advance of years, so that the psychical powers, 

 whilst themselves acquiring an increase of vigor and comprehensiveness, are 

 brought more and more under the control of the individual, and can be 

 utilized in any way in which he may choose to employ them. Thus with a 

 diminishing mobility of thought and excitability of feeling, the Mind becomes 

 more and more capable of sustained and deter minutely concentrated activity; 

 and is at the same time progressively acquiring that store of familiar ex- 

 periences, which not only constitutes the basis of all attainments in special 

 departments of knowledge, but supplies (when judiciously used) that " com- 

 mon sense " by which we form most of our judgments and direct most of our 

 conduct. During this period, moreover, the Muscular apparatus of Animal 

 life, whose actions are at first purely automatic, is brought more and more 

 under the direction of the Mind, so as to express its ideas, its feelings, and 

 its volitions. And it is whilst this transference is going on, that new habits 

 of action are most readily formed, and when once formed, are durably im- 

 pressed upon the organism ( 510, 545, 679). The excess which must exist, 

 during the whole of this period, in the constructive over the destructive activity, 

 and the large amount of the latter which (as already shown) arises out of the 

 very nature of Growth, in addition to that which proceeds from the increased 

 activity of the Animal functions, necessitates a much larger proportion of 

 repose than suffices for the adult ; but this necessity diminishes with the 

 progress of years, for the reasons already mentioned ; and thus we find that 

 whilst the young child passes 16 or 18 hours a day in sleep, half that time 

 suffices for the youth just entering on manhood. 



870. The Second Dentition, consisting in the replacement of the deciduous 

 or " milk " Teeth by the permanent Teeth that succeed them, which is the 

 most important developmental change that occurs during the period of Child- 

 hood, normally commences in the 7th or 8th year: the germs of the new teeth, 

 however, are formed long previously, having their origin in a process of gem- 

 mation from the tooth-sacs of the temporary teeth, which takes place at a 

 very early period in the development of the latter. The three permanent 

 Molars on either side of each jaw, however, have no such origin : since they 

 do not replace temporary teeth. The first pair, which usually make their 

 appearance behind the temporary molars, either contemporaneously with, or 

 a little anteriorly to, the first shedding of the deciduous teeth, are really 

 "milk" teeth, so far as their origin is concerned, since they are developed 

 from primitive tooth-sacs: on the other hand, the second true molars, which 

 afterwards come up behind them, are evolved from tooth-sacs which hold 

 the same relation to those of the first, as the tooth-sacs of the other perma- 

 nent teeth do to those of the deciduous teeth which they replace ; and the 

 third true molars, or denies sapientice, bear the like relation to the second. 

 Although the eruption of the true molars is so long postponed, yet the 



