THE EDUCATION OF THE NERVOUS SYSTEM. 339 



in the strength, accuracy, and readiness with which they 

 respond. 



Such is the development from the standpoint of the 

 increasing organisation of the system at large. Among 

 the sensory and central constituents are changes of a 

 different sort. In the very young the mental processes 

 are limited by the fact that memory is very poor. As 

 this power increases it becomes possible to hold the 

 mental image for a longer time, until finally a large 

 fraction of the mental operations involves the employ- 

 ment of such images which may be present in the 

 terms of any sense. Thus the child first receives 

 impressions which are quickly lost, then those which 

 are remembered for a time, and finally he accumu- 

 lates a store of memories which enter as modifying 

 factors into all subsequent mental activity. In advanced 

 age the mental activities have it in common with those 

 of childhood that the images are easily mislaid, but 

 when available during the period of senescence they are 

 more complete than during the earlier stages of growth. 



The study of sensory aphasia, designating by this a 

 loss of the power of recognition or expression conse- 

 quent on injury to the sensory or central elements, 

 has helped much to the understanding of the manner 

 in which our ideas are built up during the formative 

 period. Analysis indicates that the notion of a brass 

 bell is built up from its smell, taste, temperature, 

 weight, shape, colour, and the sound it makes, though 

 the information obtained through the eye and the ear 

 is that on which we commonly rely ; but a blind-deaf 

 person would emphasise the other sources of infor- 

 mation. When an impression has been received from 

 a bell it may be indicated on our part by a sound : 

 the name or an imitative noise, or a gesture of some 

 sort: a picture or the written 'word. In the ordinary 



