542 The Outline of Science 



brain. It is through the senses that all the materials with which 

 the mind builds up the higher forms of experience memory, 

 imagination, and thought are obtained. For the senses are the 

 Gateways of Knowledge. 



It would be going beyond the scope of our subject to describe 

 fully the evolution of our various organs of sense the mechanism 

 of the eye, the ear, and so on. By these instruments we are able 

 to image and focus the world outside of us. 



A sensation depends on some physical influence, the stimulus 

 affecting some part of the outer or inner surfaces, or tissues of 

 the body. In most cases there is a special organ adapted to 

 receive the stimulus, and so to transform its action into a nerve- 

 impulse for transmission to the brain, such as the eye, the ear, 

 parts of the skin, and so on. 



The acquisition of the sense of sight vastly enlarged the 

 horizon and widened the mental range; and so with hearing, 

 which is the most recently acquired of our specialised senses. We 

 know that the senses are not infallible; they are limited and im- 

 perfect; but there is no evidence whatever that the development 

 of our senses has reached finality. 



The Brain 



The structure of the brain was briefly referred to in the 

 section dealing with physiology. We need recall only that there 

 are several main divisions of the brain, each with its own peculiar 

 functions. The brain proper consists of the cerebrum, or larger 

 brain, which occupies the whole of the upper and front parts of 

 the cavity of the skull. It is divided into two great cerebral 

 hemispheres, right and left, which are linked together by 

 numerous nerve-fibres. The outer surface or cortex of the fore- 

 brain is the seat of sensation and volition. It is a wrinkled or 

 convoluted fold of grey cellular matter, which if smoothed out 

 would cover a little over a foot and a half square. There are in 

 the convoluted part of our fore-brain (the cerebral cortex) five 



