THE RECEPTORS AND EFFECTORS 73 



From these illustrations it is plain that the sensory equipment 

 of the human body is adapted to respond directly to only a 

 limited part of the environing energy complex, the remainder 

 having little, if any, practical significance in the natural environ- 

 ment of primitive man. During the progress of the develop- 

 ment of human culture mankind has very considerably wid- 

 ened his contact with the environment by artificial aids to his 

 sense organs. The range of vision has been extended by the 

 microscope and the telescope, and of hearing by the micro- 

 phone and the telephone. The photographic plate enables him 

 to extend his knowledge of the solar spectrum beyond its visible 

 limits, and the Marconi wireless apparatus brings the Hertzian 

 electric waves under his control and thus enables him to 

 put a girdle round about the earth in less than Puck's forty 

 minutes. 



We may conceive the body as immersed in a world full of 

 energy manifestations of diverse sorts, but more or less com- 

 pletely insulated from the play of these cosmic forces by an 

 impervious cuticle. The bodily surface, however, is permeable 

 in some places to these environing forces and in a differential 

 fashion, one part responding to a particular series of vibrations, 

 another part to a different series, much as the strings of a piano 

 when the dampers are lifted will vibrate sympathetically each to 

 its own tone when a musical production is played on a neighbor- 

 ing instrument. The sense organs, again, may be compared 

 with windows, each of which opens out into a particular field so 

 as to admit its own special series of environmental forces. In 

 each species of animals these windows are arranged in a charac- 

 teristic way, so as to admit only those forms of energy which are 

 of practical significance to that animal as it lives in its own natu- 

 ral environment. The sensory equipment of the human race was 

 thus established by the biological necessities of our immediate 

 animal ancestors, and there is no evidence of subsequent im- 

 provement in these physiological mechanisms or of any increase 

 in the number of our senses during the advancement of human 

 culture. What the progress of science has accomplished is to 

 supplement the limited sensory equipment of primitive man by 

 various indirect means. To recur to our analogy of a house with 

 many windows, we have not been able to increase the number of 



