NERVES 2/1 



hesitates before giving morphine, knowing that to relieve 

 the pain is to mask the danger signals so that he cannot 

 judge of the real cause of the trouble. 



Tickling is a sensation between touch and pain. It is produced in 

 parts which are poorly supplied with nerves of touch, as on the back or 

 the neck. At first, tickling is a pleasant sensation, but if continued, 

 it becomes extreme suffering. Some persons and animals who are able 

 to endure great pain are unable to control themselves when tickled. 



Itching is a sensation which is overcome by producing a greater 

 sensation in the part, as by scratching. Although itching is usually 

 only an annoyance, in a greater degree it is a torment even worse than 

 pain, and may lead a person to injure the skin seriously by deep 

 scratching. 



488. The muscular sense. Sensations of weight or of 

 resistance are judged partly by the amount of muscular 

 effort needed to move the body, and so depend in large 

 part upon the motor nerves. But the feelings of pressure 

 upon the body and of muscular effort aid in producing the 

 sensation. An object lifted seems distinctly heavier if its 

 weight is increased only ^y, while if it is placed upon 

 the skin, its weight must be increased \ before it feels 

 heavier. 



489. Necessity of epithelium. The covering of epithelium 

 not only protects the nerves from injury, but also modifies an impulse 

 which produces a sensation, so that it is spread over a larger area of 

 nerves and is made a gentle instead of a painful sensation. 



490. Motor nerves. Besides touching a sensory nerve, 

 each cell probably communicates with a motor nerve also. 

 Motor nerves begin at the central nerve cells and end at 

 the cells of the body. Over them the central nerve cells 

 send orders based upon information brought by the sen- 

 sory nerves. Many orders are sent by willful efforts of a 

 person, but by far the most are sent without our knowledge. 



