412 MESSRS. F. GOTCH AND V. HORSLEY 



SECTION 2. GENERAL PLA.N AND RESULTS OF PRELIMINARY EXPERIMENTS. 



The experiments to be referred to in this chapter were made upon both Monkeys 

 and Cats, and all involved the following operative procedure. The spinal cord of the 

 etherised animal was exposed in the lower dorsal region ; it was then divided and 

 prepared on the peripheral side of the section for 4 centims., so that the upper end of 

 this lower fragment of cord could be raised from its canal and attached to the cables 

 of the non-polarisable electrodes as described in the previous sections. The galvano 

 meter circuit was then brought by one electrode into relation with the cross section 

 of the cord, by the other with a ring of longitudinal surface at about a centimetre 

 distance. (See Plate 29.) 



The two sciatic nerves were now exposed and divided, so that they could be raised 

 and the central end placed, when desired, upon a pair of platinum electrodes (see 

 p. 301 and fig. 3), precautions being observed in their preparation with respect to 

 maintenance of circulation, drying, &c., as indicated at length in Chapter III. 



If the large and persistent resting electrical difference between the cross section and 

 surface of the cord be balanced, and the galvanometer needle thus brought into its zero 

 position, then on exciting the central end of either sciatic nerve for 5 seconds with 

 the interrupted induction current (Helmholtz side-wire), an electrical change occurs in 

 the cord, the surface contact becoming gal vanometrically negative to the cross section. 



On the cessation of the excitation, the needle rapidly returns to its original point 

 and, in most cases, then continues to slowly move to a position upon the opposite side 

 of the zero, thus indicating that the excitatory negative change only lasts as long as 

 the excitation, but is succeeded by a more lasting after-effect of opposite sign. This 

 increase in the resting difference has been already alluded to in Chapter IV. Errors 

 due to any electrical escape are mimimised by the nature of the excitation, and the 

 position of the galvanometer electrodes (see Chapter III., Section 4) ; moreover 

 convincing proof that the effect is truly an excitatory one is afforded by the following 

 facts : 



(1.) A similar negative change occurs when, with the sciatic nerve uncut, the skin 

 over the foot is pinched with ivory or touched with a hot glass rod ; 



(2.) A sudden change of small amount, but similar in sign, is produced when the 

 nerve is mechanically excited by a ligature, cut, &c. ; 



(3.) Very pronounced effects of similar sign occur when, after injection of a few 

 drops of a 1 per cent, solution of strychnia acetate, the nerves connected with the 

 lower part of the cord are mechanically excited. 



The electrical change in the cord is thus excitatory, and its resemblance to that in 

 nerve warrants the presumption (see p. 277) that it is due to the passage of excitatory 

 impulses along nerve fibres present in that portion of the cord with which the 

 galvanometer is connected. 



