LXVIIT.] REFLEX ACTION, ETC. 321 



6. Action of Potassium Chloride or Bromide or Chloral. 



Prepare a reflex frog as in Lesson LXVIII. 1. Test the latent period 

 with dilute sulphuric acid, 0,2 i)er cent., until constant results are obtained. 

 Inject 2 minims of a i per cent, solution of KCl or KBr or C2HCI3O, and 

 after ten minutes time again test the latent ])eriod. Within a short time the 

 latent period will be greatly prolonged. Plot a curve of the results, the 

 abscissa to mark time and the ordinates the length of the latent period. 



7. Electrical Stimulation. 

 (A.) Single Induction Shocks. 



(a.) From the secondary coil (key interposed) apply two fine 

 wire metallic electrodes, in the form of two loops, to the skin of the 

 leg, the electrodes being about .5-1 cm. apart. 



{h.) Stimulate with different strengths of current. No reflex 

 response. A single induction shock does not discharge a reflex 

 movement. 



(B.) Repeated Shocks. 



(a.) Leave the electrodes in situ, but adjust the coil for repeated 

 shocks. On applying a succession of even feeble shocks, a reflex 

 response is readily obtained. Make a table of the results obtained. 



{/>.) Expose the sciatic nerve without injuring the adjacent parts ; 

 on stimulating the skin of the foot or leg as before, a reflex response 

 is readily obtained, but on stimulating the sciatic nerve directly 

 under the same conditions, there may be no response until the 

 current is made distinctly stronger. This result is explained (?) by 

 stating that the peripheral terminals are more excitable than the 

 nerve trunk, while others assume that in the sciatic nerve, besidas 

 excito-motor (reflex) fibres, there are nerve-fibres which inhibit the 

 action of such fibres. It is said that very strong stimulation of 

 cutaneous nerves also excites the reflex-inhibitory lilires. 



(''.) Isolate any one of the nerves traversing the dorsal lymph- 

 sac of a frog, but leave a small square of skin attached corresponding 

 to the terminals of the nerve. Apply repeated shocks directly to 

 the nerve, in all prolmbility there will be no reflex response, but if 

 the skin be touched with dilute acetic acid, response will probably 

 take place. If, however, strong sulphuric acid be applied to the 

 skin, there will be no response. 



8. Knee-Jerk. 



(^/.) Sit on a chair and cross the right leg over the left one. 

 With the tips of the fingers or a percussion-hammer strike the 

 right ligamentum patellae. The riglit leg will be raised and thrown 

 forward with a jerk, owing to the contraction of the quadriceps 

 muscle. An appreciable time elapses between the striking of the 

 tendon and the jerk. The knee-jerk is almost invariably absent in 

 cases of locomotor ataxia, while it is greatly exaggerated in some 



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