288 



ELEMENTS OF PHYSIOLOGY 



the body and cause them to act. Transmitting impulses 

 is the whole duty of nerves. The endings of the nerves 

 are so abundant in the skin just beneath the epithelium, 

 that the point of a fine needle cannot enter without causing 

 pain. They are more numerous there than in any other 

 part of the body. Sensory nerves are not so abundant in 

 the muscles and internal organs as in 

 the skin, so that a cut gives most pain 

 as it enters the skin and may be con- 

 tinued into deeper parts with only 

 slight pain. Motor nerves begin at 

 the central nerve cells and end in the 

 cells of the body. Motor nerves and 

 motor impulses are for three pur- 

 poses : for controlling motion, secre- 

 tion, and growth. 



526. Special Sensations. When 

 something outside of the body acts 

 upon the nerves, it produces a feel- 

 ing or sensation by which the mind 

 forms ideas of the surroundings of 

 the body. The meanings of these 

 feelings must be learned when first 

 stained with Chiorid of perceived, and they are mostly learned 

 in childhood. The special senses are 

 of such importance that all except 

 the sense of touch will be included 

 in a separate chapter. 

 527. General Sensations. The cells of the body are 

 constantly sending impulses to the central nervous system, 

 signifying their needs. These impulses give rise to feelings 

 to which the mind assigns no definite location in the body ; 

 hence they are called general sensations. Such feelings as 

 hunger, thirst, fatigue, and sleepiness belong to this class. 

 They are as well understood by a baby or any young 



FIG. 196. Tactile Cor- 

 puscle within a Papilla 

 of the Skin of the Hand, 



Gold. 



n, two nerve fibers passing to 

 the corpuscle ; a, a, ramifica- 

 tions of the axis cylinders 

 within the corpuscle. 



