150 EXPERIMENTAL PHYSIOLOGY. 



the abdominal viscera. With the left hand seize the upper 

 end of the divided spinal column, and with the right the 

 skin covering it, and pull. At once the whole lower end of 

 the trunk and the lower limbs are denuded of skin. 



(c.) Take the thigh muscles between the thumb and fore- 

 finger of the left hand, and with the point of one blade of a 

 pair of scissors tear through the fascia between the biceps 

 and semi-membranosus to expose the sciatic nerve, and then 

 proceed as directed in 1. 



2. Stimuli may be classified as follows : 



(1.) Mechanical e.g., cutting or pinching a nerve or 

 muscle. 



(2.) Chemical e.g., by dipping the end of a nerve in a 

 saturated solution of common salt. 



(3.) Thermal e.g., applying the end of a heated wire to- 

 the nerve. 



(4.) Electrical 



(a.) Continuous current. 

 (6.) Single induction shocks, 

 (c.) Interrupted current. 



3. Stimulation of Muscle and Nerve. It is convenient to 

 modify somewhat the physiological limb, in order to render the 

 muscular contraction more visible. Apparatus. Frog-pithing 

 needle, scalpel, scissors, forceps, straw-flag, pins, muscle-forceps, 

 camel's-hair brush, saturated solution of common salt in a glass 

 thimble, ammonia, copper wire, spirit lamp or gas flame. 



4. Mechanical Stimulation. 



(a.) Pith a frog, destroying its brain and spinal cord. 

 Remove the skin, and proceed as directed by the method 

 (Lesson XXVII., 1) for preparing a nerve-muscle prepara- 

 tion, as far as the isolation of the sciatic nerve, but modify 

 the subsequent details as follows : 



(6.) After the nerve is cleared as far as the spine, clear all 

 the muscles away from the femur, and divide the latter 



