nation as to the relation between the angle of crossing of the coil- 

 a\e- and the strength of the shocks. 



\~n. '1 he re:-ersiiii/ (or roekiny) key or pule eliaiit/cr. Xote the 

 inechani-m of the reversing key and draw diagrams showing the 

 connections to he made in order to n-e it (l) in reversing the direc- 

 tion of the current ; (2) as a double key without changing the wire- ; 

 I 3 ) a- a single key. 



i Si. Kxamine the non-polarizable electrodes. They are -oaked in 

 phy-ioln^ieal salt solution, then tilled with ZnS< ) 4 solution in 

 which is immersed a Zn rod. Take great care not to spill ZnS< ), 

 on the outside of the boots. Zinc is dissolved at the anode ( -(- pole ). 

 and i- deposited on the 7n rod at the cathode (- - pole). The Xa 

 and (.'1 ions carry the current through the tissue. 



Set up the non-polarizable electrodes, place on litmus paper mois- 

 tened with physiological -alt >olmion, and determine the effect of 

 pas.-ing a current through them. Does electrolysis take place? Do 

 you see why they are used in phy-iol gy ? 



Immediately after using, wash out the ZnS( ), very carefully and 

 place the electrodes in physiological salt solution to soak. Wipe off 

 the Zn rods so that they will be ready for another experiment. 



I!. PHENOMENA OF CONTRACTILITY AXD IRRITA- 

 BILITY. 



Muscular Contractility. Muscle cells are typically stimulated to 

 contraction by impulses conveyed through tracts of conducting 

 ti--ue called nerves. A muscle with its attached nerve represent 

 the chief motor organ of higher animals. 



i .Si. \crrc-mnsclc preparation. A muscle with nerve attached 

 (gastrocnemius-sciatic) can be isolated a- foil ws : Destroy the 

 brain and -pinal cord of a frog bv pithing, as demonstrated. All 

 -pontaueons movement -honld cease. Do you know \\liv: l\e- 

 move the --kin from the whole body of the frog except the head, as 

 secretions of the -kin injure the muscle. The object now IS to 

 remove tin- gastrocnemius muscle (.-till attached to the femur), and 

 the whole -ciatic nerve (-till attached to the gastrocnemius muscle) 

 from it- origin in the -pinal cord. Xote on the dorsal side of the 

 thijji a longitudinal depression between the va-tu- externus and 

 ?emimembranosns mu-cle- ( -ee l-.cker'- l'rg. \^. <)$). The -ciatic 



ner\c lie'- in thi- groo\-.' along with the hi 1 \e--els. Lift up the 



m-rve vi-r\- genth with a gla-s -eeker and caretullv i-olate it as 

 far a- the knee in a downward direction. 'I hen -eparate well the 



48 



