86 EXPERIMENTAL PHYSIOLOGY 



vary, however, according to the degree of stimulation. Place the peripheral 

 end upon a fine pair of electrodes and stimulate. There is no muscular con- 

 traction. 



(ii.) Place a second ligature round another posterior root, this time tying 

 as far from the cord as possible, and cut through the nerve peripherally to the 

 ligature. Stimulate the central end, i.e. the part still attached to the cord. 

 The limbs are thrown into convulsions, more or less marked and extensive 

 according to the strength of the stimulation. These two stimulations prove 

 that the posterior roots contain no efferent, but do contain afferent 

 fibres. 



(iii.) Cut through all the posterior roots of that side. Any mechanical 

 stmralation to the skin no longer produces movements of the legs, though 

 stimulation of the skin of the opposite leg will produce movements in 

 both. 



(iv.) The section of the posterior roots brings the anterior into view. By 

 placing ligatures round two of these and tying (a) near the cord and (&) near 

 the junction with the posterior root show that stimulation of the peripheral 

 end leads to contraction of muscles, but stimulation of the central end produces 

 no effect. The anterior root therefore contains efferent but no afferent 

 fibres. 



(v.) Cut through all the anterior roots, and then show that no movement 

 of that leg can now be produced by stimulation of the skin of the leg of the 

 opposite side. 



STIMULI which affect a nerve may be classified as follows : — 



1. Thermal. 



Experiment 2. — Make a nerve muscle preparation and touch the 

 end of the nerve with a copper wire which has been heated for a few 

 moments in a Bunsen flame. The muscle contracts. That heat should 

 act as a stimulus it is necessary that the temperature should be high. 

 If heat be gradually applied it will kill without stimulating. 



2. Mechanical. 



Experiment 3. — To produce mechanical stimulation cut through 

 the nerve used in the previous experiment just below the point to 

 which the hot wire was applied. The muscle contracts, thus showing 

 that the mechanical process of cutting has stimulated the nerve. In 

 the next place, pinch the upper end of the nerve and show that this 

 also acts mechanically as a stimulus. Show also that the nerve can 

 be stimulated by giving it a sharp tap with the edge of the handle of 

 a scalpel, thus pinching it between the scalpel and the solid support 

 upon which it rests. 



3. Chemical. 



Experiment 4. — Take a nerve-muscle preparation and lift up the 

 nerve on a glass rod bent as a hook, so that the cut end of the nerve 

 hangs down. Touch the cut surface of the nerve with a drop of a 

 •8 per cent. KOH solution in normal saline, held in a watch-glass. 

 With each contact the muscle contracts, and, if the nerve be immersed 

 a little time, passes into an irregular tetanus. Cut away the piece 



