CHAPTER XVIII 

 ORGANS OF SPECIAL SENSE 



Organs of special sense are the receptor's of the body, receiving 

 stimuli and passing them on to the sensory neurones. Through 

 the latter, connection is made with the ganglia and nuclei of the 

 sympathetic and central nervous systems. As the essential ele- 

 ment of the receptor is the nerve ending, the sense organs are 

 ectodermal in origin; but the majority become surrounded by 

 complex accessory structures derived from the mesodermal con- 

 nective tissue. 



Receptors are located in the deeper tissues of the body (striated 

 and smooth muscles, mesenteries and cavities of the joints) 

 and in the outer covering. The latter are more numerous and in- 

 clude the more complex sense organs. These may be either in 

 the epidermis or embedded in the mesodermal supporting layers 

 of the skin. There is, during the course of evolution, a general 

 tendency toward assuming a deeper position. 



A direct correlation exists between the diverse stimuli re- 

 ceived and a specialization of the receptors to become specific 

 for certain types of stimuli. Thus the vertebrates have proprio- 

 ceptive organs which respond to pressure; tactile cells or cor- 

 puscles; pain corpuscles; and the more specialized organs which 

 receive chemical, or light and sound wave stimuli. The latter 

 tend to be collected into close packed groups, while the skin 

 and visceral endings are usually isolated corpuscles and are 

 known as the nerve end apparatus. 



Nerve endings. The simplest, and perhaps most primitive, type 

 is the free nerve termination. The sensory fiber loses its myelin 

 sheath and breaks into fine fibrils which ramify into the muscle 

 or skin. This type is typical of striated muscle endings. Free 

 terminations in the skin are more typical of the water living 

 vertebrates than of the land living animals. 



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