Nervous System and Sense Organs 213 



and cold ; the second group includes those such as the eye and ear which 

 may respond to stimuli originating at great distances. 



The Exteroceptors of the Skin. — Located in the dermis of the 

 skin are the special receptors for pain, pressure, heat, and cold. These 

 are scattered over the entire surface of the body, but are more con- 

 centrated in some areas than in others. Thus the lips are most sensi- 

 tive to heat, the tips of the fingers to touch, and the back of the neck 

 is relatively insensitive. 



Each individual receptor can receive only one type of sensation, 

 although a single stimulus may evoke a reaction on the part of more 

 than one receptor. For example, a very hot object may stimulate both 

 the pain and heat receptors. 



Fig. 66. — Various sense receptors. A, Free nerve ending; B, encapsulated 

 nerve ending; C, a taste bud; D, portion of olfactory epithelium. 



The distribution of these receptors over the body surface can be 

 mapped; for example, a bristle when lightly touched to the skin will 

 be noticeable in some areas and not in others; similarly a heated object 

 will be detected only in certain specific spots. By careful study, it has 

 been demonstrated that the receptors for pain and heat are simply un- 

 myelinated free nerve endings scattered throughout the upper layers 

 of the dermis (Fig. 66,A). The endings for the detection of cold and 

 touch are enclosed in special capsules (Fig. 66,B). 



The Inter oceptors. — Normally, an individual is not aware of 

 sensations initiating in the visceral organs. Occasionally, however, ex- 

 treme pain is experienced. In general, this pain is not nearly so local- 

 ized as in the cutaneous sensation. The exact manner in which this 

 pain occurs is not entirely clear, but it has been suggested that the pain 

 fibers are not very abundant in the visceral organs, thus any stimulation 

 of them would have to originate from a fairly large area. 



