xvii THE SENSE ORGANS 321 



CHAPTER XVII 



THE SENSE ORGANS 



THE sense organs are the means through which stimuli are 

 received from the outer world. They are always connected 

 with the central nervous system by nerve fibers which carry 

 the impulses derived from the various objects in the envi- 

 ronment which act upon the organism. The sense organs in 

 general are specialized so that they are exceedingly sensitive 

 to certain forms of stimulation, and but very slightly so to 

 others. The eye is affected ordinarily only by light, the 

 auditory organ only by sound, the olfactory organ only by 

 contact with certain substances. The specific functions of 

 the sense organs are determined by their structure, each 

 being specialized in relation to some particular source of 

 stimulation from the outside world. 



Sense Organs of the Skin. - The skin is richly supplied 

 with sensory nerve endings both in the epidermis and in the 

 corium. The epidermis is supplied by fine fibers which re- 

 peatedly branch and form a sort of network between the cells. 

 The free ends of the ultimate branches are often enlarged 

 into a disk (Retzius). The corium contains special end 

 organs, the so-called touch corpuscles, which lie under small 

 papillae of the epidermis. Each touch corpuscle is com- 

 posed of a small heap of flattened cells between which are 

 found the terminal branches of the nerve with which each 

 organ is supplied. The temporary papillae of the female of 

 Rana fusca, according to Huber, possess a structure in 

 some respects resembling that of the touch bodies. 



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