LABORATORY WORK 



199 



the terminals of the galvanometer so as to be able to adjust the 

 sensibility for various purposes. If the direction of the current in 

 the slide-wire is not such as to oppose the E. M. F. of the tissue, 

 the connections of the battery must be reversed, or a commutator 

 may be interposed in the circuit. 



The skin of the frog is the simplest secretory structure in which 

 the electrical change of activity can be seen. The electrodes are 

 placed one on each leg between the knee and the ankle. The 

 sciatic nerve is prepared on one side, a ligature tied around it, and 

 the nerve cut on the central side of the ligature. It is laid on two 

 wires, best of platinum, connected with the secondary coil of the 

 induction apparatus and the current 

 thrown in when the galvanometer 

 has been brought to zero. The 

 nerve should be raised out of the 

 wound and laid on the stimulating 

 electrodes held in some support. A 

 pillar of plasticine is the simplest. 

 Since the skin of one leg only is 

 stimulated, that of the other acts 

 merely as a conductor. If both 

 nerves were stimulated, the changes 

 in one would neutralise those in the 

 other, owing to the opposite direc- 

 tions in which they are connected 

 to the galvanometer. The actual 

 electrical change seen is sometimes 

 of a complicated nature, consisting of 

 more than one phase. The explana- 

 tion of the whole is not altogether 

 clear, but we need only observe the 

 fact of a change in the gland cells. 



Since the stimulation of the sciatic nerve causes the muscles to 

 contract, and this would interfere with observation of the glandular 

 effect, we must first give the frog an injection of the arrow poison, 

 curare, by means of a hypodermic syringe inserted under the skin 

 of the back. About 3 drops of a 0.05 per cent, solution will usually 

 be sufficient, but different samples of curare vary in strength. The 

 solution should be made*fresh. In small doses it prevents stimula- 

 tion applied to the nerve from reaching the muscle, but does not 

 affect the glands. When the frog is completely paralysed it is 

 pithed or beheaded. 



The sciatic nerve is found by making an incision through the 

 skin between the end bone of the spinal column, urostyle, and the 

 pelvic arch on one side at the extremity of the former (as in Fig. 18), 



FIG. 18. Position of the 

 Sciatic Nerve. 



a, place in which the incision 

 is made. 



