46 EXPERIMENTAL GENERAL PHYSIOLOGY 



metallic slider (S) the circuit has two paths presented: 1st, from S 

 direct to B; 2d, from S through G and back to B. The total current 

 is divided into two parts : C, which passes along the wire from S to B, 

 and C', the derived circuit which passes through the galvanometer. 

 Suppose the resistance to the last-named current is R' and that to 

 the direct current is R, the relative strength of these two currents 

 is expressed in the following proportion: C f : C : :R:R'. 



But the resistance of the German-silver wire may be conveniently 

 divided into 100 equal parts (100 r). 



If the slider be placed at any position along the wire, say at X cm. 

 from the end, then the formula would be C' ': C : : xr : R'. 



~, xr~ 



~R L 

 Suppose that R=l ohm (r=0.01 ohm); R' = 2 ohms and #=0; 



/-VA 



i. ., suppose the slider to hard up to B } then C"= C=0. This 



makes it clear that when the slide is in the zero position there will 

 be no current passing through the galvanoscope. 



(2) What is the relative strength of the two currents when #=10? 



(3) What is the relative strength of the two currents when #=50? 



(4) What is the relation of C" to C when #=99? 



(5) What is the relation of C' to C when #=100? 



From this course of reasoning it is evident that in the simple 

 rheocord we have an instrument with which we can vary a derived 

 current from zero to a maximum. Just what the value of this derived 

 current will be will depend upon the voltage of the cell or battery 

 and the total resistance to be overcome, as well as upon the distribu- 

 tion of that resistance. 



(6) Verify the theory just developed, making a table of galvano- 

 scope readings. 



X. MUSCLE-NERVE PREPARATION. 



(a) The Classic Muscle-nerve Preparation. 



1. Appliances. Frog board and pins; operating case; glass nerve 

 hooks, like Fig. 28, A, made as follows: take a 10 cm. piece of 

 glass rod, heat and draw in center to about 1J mm. diameter; cool, 

 cut in two, heat the points to smooth them, and bend the end over 

 to form the hook. 



Simple myograph or muscle lever (Fig. 26). Watch-glass with salt 

 crystals; 20 cm. of thick copper wire. 



2. Preparation. Pith a frog and fix to frog board, with dorsum 

 up. It will be taken for granted that the student is familiar with the 

 anatomy of the frog's leg and thigh. The accompanying cut (Fig. 

 27) may serve to refresh the memory. 



