842 THE NERVOUS SYSTEM 



The exteroceptive or general cutaneous field, having sense organs detecting 

 stimuli at or near the surface of the body, and (2) the proprioceptive field, 

 with sense organs located in the body-wall tissues, such as striated muscles, 

 tendons, joints and the equilibration structures of the internal ear. 



The visceral sensory organs receive stimuli from the interoceptive field, 

 that is, the visceral structures of the body. 



2. Somatic Sense Organs 

 a. Special Somatic Sense Organs 



The visual organs, the ear, and in water-living vertebrates the lateral-line 

 system, are sense organs of the special variety. 



b. General Somatic Sense Organs 



These structures are in the form of free nerve endings, terminating among 

 cells and around the roots of hairs, or they are present as encapsulated nerve 

 endings such as the corpuscles of Meissner, end bulbs of Krause, and Pacinian 

 corpuscles (fig. 359A-D). 



3. Visceral Sense Organs 



a. Special Visceral Sense Organs 



The taste buds of various sorts, located generally on the tongue, mucous 

 surface of the buccal cavity and pharynx and in some fishes on the external 

 body surface are specialized visceral sense organs (fig. 285E). 



In most craniates the paired olfactory organs are exteroceptive in function, 

 although, possibly, olfactory organs may be regarded as primitively intero- 

 ceptive. The olfactory organ is regarded generally as a special visceral sense 

 organ. 



b. General Visceral Sense Organs 



General visceral sense organs are located among the viscera of the body. 

 They represent free-nerve endings lying in the walls of the digestive tract and 

 other viscera. They respond to mechanical stimuli. 



4. The Lateral-line System 



The lateral-line organs are a specialized series of organs located in the 

 cutaneous areas of the body. They are found in fishes and water-living am- 

 phibia. A sense organ of the lateral-line system is composed of a patch of 

 hair cells or neuromasts, columnar in shape, possessing cilia-like extensions 

 at the free end (fig. 359E). Basally the hair cells are associated with the 

 terminal fibrillae of sensory nerves. The hair cells are supported by elongated, 

 sustentacular elements. In cyclostomous fishes the neuromasts are exposed 

 to the surface, but in Gnathostomes they lie embedded within a canal system 



