108 GENERAL CONCEPTS 



organs which sense orientation in the field of gravity); distance receptors 

 report on objects away from the body (sight, smell and hearing), and 

 interoceptors provide sensations of pain, fullness, and so on from 

 internal organs. 



When a sense organ is stimulated continuously it may either give 

 off a continuous stream of nerve impulses or it may quickly cease re- 

 sponding to tlie stimulus. The proprioceptors of the body are generally 

 of the first type, nonadaptive, whereas the exteroceptors are generally 

 adaptive, and soon become nonresponsive to a continuing stimulus. 

 The advantage of sense organ adaptation is clear: it prevents a continual 

 train of nerve impulses nnpinging on the brain from all the body's 

 sense organs, yet does not interfere with the body's responding to changes 

 in the pattern of stimuli which are likely to be important for survival. 



The actual excitation of the sensitive cells of the sense organ is 

 either via mechanical stress, via chemical stimulation by contact of the 

 molecules of some substance from the environment, or via some chemical 

 process induced in the sense cell by the stimulus. An example of the 

 latter is the chemical reaction induced by light falling on the sensitive 

 cells of the retina of the eye. 



The functioning of a sense organ in animals other than man can be 

 deduced from its morphology and nerve connections. It can be investi- 

 gated by connecting the efferent nerve to an amplifier and oscilloscope, 

 applying stimuli to the sense organ, and measurnig the resulting nerve 

 impulses. It can also be investigated at the behavioral level, by training 

 the animal to associate one situation with a given stimulus and a second 

 situation with a different stimulus, and then observing its ability to 

 distinguish between the first and second stimuli as they are gradually 

 changed to resemble each other. 



Chemoreceptors. Our own senses of taste and smell can be dis- 

 tinguished, for the taste buds are organs in the lining of the mouth 

 which respond to substances in watery solution, whereas the olfactory 

 epithelium is in the lining of the nose and responds to substances which 

 enter as gases. In most lower animals, the distinction between taste and 

 smell is blurred, for chemoreceptors are found over much of the surface 

 of the head and part of the body in fish, and insects have chemoreceptors 

 in their feet. Chemoreceptors are sensitive to remarkably small amounts 

 of certain chemicals. Most people can detect ionone, synthetic violet 

 odor, at a concentration of one part in 30 billion parts of air. Certain 

 male insects can detect the odor given off by the female of the species 

 over a distance of two miles. Several thousand different odors can be 

 recognized by man, but there is no clear correlation between the chemi- 

 cal composition of a substance and its smell. 



Chemoreceptors are probably the most primitive of the distance 

 receptors, and many kinds of animals depend solely upon them for 

 finding food, avoiding predators and meeting mates. 



Mechanoreceptors. The skin of man and other mammals contains 

 several kinds of sense organs. By making a survey of a small area of skin, 

 point by point, and testing for regions sensitive to touch, pressure, tem- 

 perature and pain, it has been found that receptors for each of these 



