NERVES 275 



brain when he feels it go down to his little finger. Explain that it 

 only seems to come from the finger, but does not go there. 



1 1 8. With a power of at least 200 diameters, show specimens of a 

 nerve mounted for the microscope. In a specimen of nerve cut length- 

 wise, notice the slender conducting fiber running down the middle of 

 each thread and its thicker, clear covering of fat. In a specimen cut 

 across the nerve, notice that the conducting fiber of each nerve thread 

 appears as a central dot within a circle of the protecting fat. Notice 

 the fine connective tissue between the threads. Sketch the specimen. 



119. Show the difference in sensibility of different parts of the skin 

 by touching it with the points of a pair of compasses. Upon the balls 

 of the fingers the points will seem separate even if near together, while 

 upon the back they will seem one when separated two inches. 



120. Show that some parts of the skin feel sensations of temperature 

 more than the others by drawing the point of a lead pencil slowly 

 across the cheek. At intervals there will be felt a cold sensation, show- 

 ing that a special nerve of temperature has been touched. 



121. Cut a nerve in a recently killed frog and separate it from the 

 flesh for a short distance. After a moment, pinch the nerve, and 

 the muscles will contract. 



REVIEW TOPICS 



1. Describe the essential parts of the nervous system. 



2. Describe a nerve. 



3. Explain the two results of irritating a nerve. 



4. Define and name the kinds of sensory impulses. 



5. Describe the sensations of touch, pain, temperature, 



and weight. 



6. Tell how the cells make known their wants. 



7. Show how epithelium aids the sense of touch. 



8. Name and describe the three kinds of motor impulses. 



9. Give the rate of transmission of impulses along nerves. 



10. Describe the effects of pressure upon a nerve; of 



cutting a nerve ; and of disease of a nerve. 



11. Tell how alcohol affects a nerve. 



