338 THE GROWTH OF THE BRAIN. 



for the reactions. It is but rarely that a vanishing 

 power can be thus tracked, but there is ample sug- 

 gestion that many ancestral phases are for a time 

 exhibited and then outgrown by the maturing brain. 

 Among the developing functions sensibility comes first. 

 Indeed the sensory system in certain human monsters 

 may be well developed, although both central and 

 motor elements are quite wanting. 1 To this general 

 sensitiveness, evident early in foetal life, and without 

 which the central system ceases to be significant, there 

 are rapidly added, about the time of birth or shortly 

 after, the more special sensations of taste, smell, touch, 

 hearing, sight, and of temperature. Then, as organi- 

 sation progresses, come the emotions of fear, astonish- 

 ment, anger, closely followed by the development of 

 the intellect and will, with the power of language and 

 self-consciousness. This is not the place to essay the 

 reduction of all these various expressions, intended to 

 indicate grades of mental development dependent on 

 the organisation of the central cells, to their physio- 

 logical equivalents. Since the special senses first 

 become useful, the various reactions of the individual 

 are customarily associated with one or another of them. 

 That this is somewhat a matter of chance the education 

 of all defectives shows. The development of the parts 

 of the brain devoted to the mediation of the special 

 sensations as well as the sense organs 2 (Luckey) like 

 those of hearing and sight, is rapid at first, then slow, 

 yet continuous up to maturity, as is seen in the curves 

 for cortical growth, already given. With the refinement 

 in sense perception and the accompanying central 

 changes, comes a corresponding increase in the control 

 of the motor elements, and in these latter an increase 



1 Leonowa, Neurologisches Centralblatt, No. 20, 1894. 



2 Luckey, Am. Journ. of Psychol., 1895. 



