•±1 LIBRARY 2 



\^V MASS. . 



CHAPTER 14 

 THE SENSE ORGANS 



The sense organs of animals consist of sensory cells or groups of cells 

 adapted to respond to stimuli and transmit an impulse to the nerves. 

 The essential property of sense cells is, therefore, irritability. Like 

 nerve cells they are able to transmit impulses caused by physical and 

 chemical changes, either in the environment or within the organism. 



The receptor organ may be either the neurosensory cell itself, or a 

 secondary sensory cell which transmits a stimulus to the nerve. In 

 higher animals, moreover, sense organs, in addition to receptor cells, 

 have various mechanisms for protecting and supporting the sensory cells 

 or conveying stimuli to them so that great diversity of receptor cells 

 and of sensory nerve terminations has arisen during the course of evolution. 



The result is a considerable number of special senses. These may be 

 classified as major or minor, according to their importance. Major 

 senses are touch, taste, smell, hearing, and sight. Minor, in addition 

 to the muscular and visceral senses, include heat, cold, pain, hunger, 

 thirst, fatigue, sex, and equilibrium. It is evident that this division 

 of the senses rests on their conscious accompaniments. 



On the basis of the source of stimulus, senses are Exterior — sight, 

 hearing, taste, smell, pressure, heat, cold; or Interior — pain, hunger, 

 thirst, the muscular and visceral senses, equilibrium, lust. 



Some physiologists recognize as many as thirty-six special senses, 

 separated for the most part by differences in sensation. Certain it is 

 that we have many more than the traditional five! 



EVOLUTION OF SENSE CELLS 



Sense organs and sensory nerves alike begin in responsiveness to 

 stimuli, such as is manifested by amoeba, which reacts to changes in 

 pressure, light, heat, chemical substances, and electricity. 



The first sense cells differentiated are the neurosensory cells of coelen- 

 terates, which establish nervous connexion with underlying muscle 

 cells by means of protoplasmic processes, while the body of the cell 

 remains in the external epithelium. Each cell may have a stiff sensory 

 bristle or hair. 



An advance towards conditions in higher animals is taken when the 

 body of the cell recedes from the surface, retaining connexion by means 



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