16 SMELL, TASTE, ALLIED SENSES 



Here then must be a wholly novel set of sensory relations. 

 As to the sensations arising from these organs man can 

 form no direct conception, for they are entirely outside the 

 range of his experience. Hence Leydig, the discoverer of 

 the sensory nature of these parts, wrote of them as organs 

 of a sixth sense. Thus to the older workers the senses of 

 the lower animals were like those of a human being that 

 had suffered either curtailment or expansion even to the 

 extent of excluding or including whole categories of 

 stimuli. But quite aside from the question of the number 

 and variety of these parts, is the opinion held by most of 

 the early workers that the sense organs of the lower ani- 

 mals are primarily concerned with providing the brain 

 or corresponding structure of the given creature with that 

 body of sensation which was supposed to represent all the 

 significant changes in the effective environment. 



2. Modified View due to Theory of Reflex Action. 

 The belief that sense organs were chiefly concerned with 

 providing the brain with the elements of which the mental 

 life is composed suffered an important limitation from the 

 work of the physiologist. This limitation arose from the 

 development of the idea of reflex action. Originating 

 about the time of Descartes in the seventeenth century, the 

 conception of the reflex action grew in time into a most 

 important principle for the interpretation of nervous 

 operations. It was at first applied to that form of 

 nervous activity whose outcome is fairly constant and in 

 a way mechanical in that it is unassociated with conscious- 

 ness, but it was gradually extended to include those per- 

 formances in which consciousness is involved and at 

 present it commonly refers to any chain of nervous 

 activity in which a sensory stimulation produces an im- 



