STIMULATION THE SENSES 105 



respond is not great enough to stimulate the nerve endings, even 

 when converted into an appropriate form. It must act, therefore, 

 as a trigger, or an electrical relay, setting off some store of 

 potential energy present in the receptor mechanism. 



All evidence available goes to show that, so long as a nerve 

 fibre is stimulated at all, the process set up in it, and passing 

 as a disturbance along it, is the same in all kinds of nerves, and 

 always of the same magnitude. We have seen this to be the case 

 with muscle, and it has also been shown, experimentally, to be the 

 same with motor, efferent nerves. But the direct experimental proof 

 is yet wanting for sensory, afferent nerves. The way in which the 

 fact applies to the phenomena of sensation is expressed in the law 

 known as that of " specific sense energies," a somewhat unintelligible 

 phrase. What is meant is that, whatever the manner in which 

 a nerve connecting a special receptor with the brain is stimulated, 

 the sensation is always that associated with stimulation of this 

 organ by its appropriate form of external energy. It matters not 

 how the nerve from the eye is stimulated ; the sensation is that of 

 light. The clearest case is that of one of the nerves of taste, which 

 passes through the ear in a way accessible to stimuli. Whether 

 these stimuli be electrical, mechanical, thermal, or chemical, the 

 sensation is one of taste, and nothing else. The object of each 

 receptor mechanism is then to provide a stimulus of some sort 

 to its nerve, no matter what. All that is necessary is that the 

 arrangement shall be such that the external influence shall effect 

 a change which actuates a stimulating agency. 



The process may be illustrated thus : the nerve may be com- 

 pared to an electrical circuit which can be connected up to a battery 

 by closing a switch. It does not matter how this switch is closed. 

 But, if light be the agent, it is clear that something sensitive to 

 light must be present and be made to close the switch, say, by a 

 current produced in an electro-magnet by a photo-chemical cell. 

 If by sound, something similar to a microphone, and so on. These 

 examples are not to be understood as implying that such are the 

 actual means adopted in the eye and the ear. 



In physical measurements we can convert any form of energy 

 into an electrical current by a proper means, and in the physiology 

 of the senses any form of outside stimulus is converted into one 

 and the same form of nerve impulse. 



But, it will naturally be asked, how can we distinguish sights 

 from sounds, taste from touch, if the messages differ only as 

 regards the particular nerve by which they arrive at the brain ? 

 We here come into contact with the mysterious relation between 

 consciousness and the physiological changes in the brain. All that 

 can be said is that when a particular region of the brain is set into 



