l82 PRACTICAL PHYSIOLOGY. 



[XXX. I 



(c.) Pinch the free end of the nerve sharply with forceps; the 

 muscles contract and the straw flag is suddenly raised. Cut off 

 the dead part of the nerve, contraction also occurs. 



(d.) Prick the muscle with a needle; it contracts. 



For the purposes of the student it is sufficient to expose o ie 

 sciatic nerve in sitUy and observe the movements of the foot an 1 

 leg. 



Mechanical stimulation is rarely employed, as the part stimulated is apt to 



be injured by the stimuli. Heidenhain in 1856 devised what he called a 

 Tetanomotor for this purpose. It consisted of a Wagner or Neef s hammer, 

 with one end prolonged and carrying a small ivory hammer, which beat the 

 nerve placed under it. Recently v. UexkiiU has devised apparatus for this 

 purpose [Zeits. f. Biol., Bd. xxxi.). 



{e.) Mechanical Stimulation by removal of pressure. — Place the nerve of a 

 nerve-muscle preparation on a moist glass plate, press the nerve slowly and 

 steadily with a curved i mm. thick glass hook. If pressure be applied 

 steadily and uniformly the nerve is not excited, but on suddenly removing 

 the pressure the muscle contracts {v. UexkiiU). 



5. Thermal Stimulation. 



(a.) To the same preparation apply, either to muscle or nerve, 

 a wire or needle heated to a dull heat ; a contraction results in 

 either case. Cut off the dead part of the nerve. 



6. Chemical Stimulation. 



(a.) Place saturated solution of common salt in a glass thimble, 

 or on a glass sUde, and allow the free end of the nerve to dip 

 into it. Owing to the high specific gravity of the saline solution, 

 the nerve floats on the surface, but sufficient salt difi'uses into 

 the nerve to stimulate it. After a few moments, the joints of 

 the toes twitch, and by-and-by the whole Hmb is thrown into 

 irregular, flickering spasms, which terminate in a more or less 

 continuous contraction, constituting tetanus. Cut off the part 

 of the nerve aff'ected by the salt ; the spasms- cease. Some apply 

 finely powdered salt to the nerve, others glycerin. 



(h.) Using a similar preparation, cover the leg with the skin of 

 the frog, or wrap it in blotting-paper saturated with normal salLue. 

 Expose the fresh-cut end of the nerve to the vapour of strong 

 ammonia. The ammonia must not act directly on muscle, hence 

 the glass vessel must be placed above the nerve, and the nerve 

 raised to the ammonia. There is no contraction of the muscle, but 

 the ammonia kilk the nerve. 



Instead of doing this, the whole leg may be laid on a card, covered with 

 blotting-paper moistened with normal saline, with a hole in it just suflSeient 

 to allow the sciatic nerve to pass through it. The card is placed over a 

 test-tube containing a drop of ammonia ; the nerve hanging in the vapour 

 of the latter is speedily killed, but there is no contraction of the muscle. 

 Apply ammonia to the muscle ; it contracts. 



